The Airwaves

01What's that signal?

The short version: Search everything in this guide by what you noticed - "plane on a flight tracker", "phone says SOS only", "scanner went silent", "antenna on a light pole", "clock set itself" - and jump to the explanation. Below it, the spectrum chart lays every band in this guide on one ruler, from the time signal that sets radio-controlled clocks to the 5G millimeter waves.

The spectrum on one ruler

Each bar is a band covered in this guide, placed on a logarithmic scale so that kilohertz and gigahertz fit on the same line. Tap a bar for what lives there, where the rule applies and the source.

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02Radio in plain words

The short version: Every wireless signal around you - Wi-Fi, 5G, a taxi dispatcher, a plane's transponder - is a radio wave, and the main thing that sets one apart from another is its frequency, counted in hertz. Regulators slice the spectrum into named bands from VLF to EHF, and they match each service to a band based on how waves at that frequency travel. Lower frequencies get through walls and cover more ground with fewer transmitters; higher frequencies offer far wider channels, so they carry more data, but a pane of energy-saving glass can stop them. A shipwreck hurried along the rules on who may use which slice: the US first required radio licenses in 1912, after the Titanic, and created the FCC in 1934.

What is a radio frequency, and what does "hertz" mean?

What you see: A number like "2.4 GHz" or "6 GHz" on a Wi-Fi router box, or "5G" on your phone's status bar.

A radio wave is an electromagnetic wave, the same family as light but far lower in frequency. The German physicist Heinrich Hertz is credited with having "solidified and proved the theory of electromagnetic waves," the work that inspired Guglielmo Marconi to build a wireless business.³

Frequency is how many times the wave repeats each second. One cycle per second is one hertz (Hz). Radio frequencies are large numbers, so they come with prefixes: a kilohertz (kHz) is a thousand cycles per second, a megahertz (MHz) a million, a gigahertz (GHz) a billion. A "2.4 GHz" Wi-Fi radio is putting out a wave that repeats about 2.4 billion times a second.

The unit of frequency, the hertz, is named after Heinrich Hertz.⁴ Working in a lecture hall of the technical university in Karlsruhe, Germany, he observed the propagation of an electromagnetic wave for the first time on November 11, 1886, and his measurements showed that the effect traveled at the speed of light.⁴

Wavelength is the physical length of one cycle of the wave, from one crest to the next. Frequency and wavelength move in opposite directions: the higher the frequency, the shorter the wave. That relationship is why engineers talk about "millimeter waves" for some 5G bands and why antennas for low frequencies are long while the antenna inside a phone is tiny.

The rule that ties them together: frequency times wavelength equals the speed of light, about 299,792 km per second.⁵ Dividing the speed of light by the frequency gives the wavelength, so a 100 MHz wave, in the middle of the FM band, is about 3 m (10 ft) long, and a 3 GHz wave is about 10 cm (4 in) long.

When a regulator talks about a band, it means a range of frequencies with a lower and upper edge, such as 3.7-3.98 GHz. The width of that range, 280 MHz in that example, is the band's bandwidth.²

What do VHF and UHF stand for?

What you see: "VHF/UHF" printed on a TV antenna box, a marine radio or a handheld scanner.

They are band names. US federal rules divide the radio spectrum into numbered bands, each ten times wider than the one before, and give each a symbol and an older "metric" name that hints at the size of its waves.⁶ VHF is "very high frequency" and UHF "ultra high frequency."

Source for every row: 47 CFR 2.101.⁶

Each band includes its upper limit but not its lower limit, so exactly 300 MHz belongs to VHF, not UHF.⁶ The metric names line up with the wavelengths: "metric" waves (VHF) are measured in meters, "centimetric" waves (SHF) in centimeters, "millimetric" waves (EHF) in millimeters.

Words you hear in advertising, such as "microwave" or "mmWave," are not in this table. They are industry shorthand, and they do not always match the official band edges.

Why do low frequencies reach farther?

What you see: An AM station that comes in from far away at night, or a phone that keeps one bar deep inside a parking garage when the 5G icon has disappeared.

Australia's spectrum regulator, ACMA, notes that "the way a radio wave spreads or propagates depends on its frequency," and matches services to bands on that basis.⁷ Regulators everywhere work the same way, because frequency changes how a wave behaves on its way to you.

The broad pattern: lower-frequency waves lose less energy going through walls, bend around obstacles more readily and cover a wide area from one transmitter. That is why a low-band cell site can blanket a large area, and why services that must reach everyone, everywhere, tend to sit lower in the spectrum. The building figures in the next item show the wall part of this in numbers.

Britannica describes how this works at the low end of the spectrum. Below 3 MHz, which takes in the LF and MF bands, surface waves travel along the ground "as if in a waveguide" and can cover very large distances.⁸ In the HF band, the main mode is reflection off the ionosphere: by day its low D layer absorbs HF waves, but at night the D and E layers often disappear, so waves reflect off the higher F layer and reach much farther.⁸ The broadcast section covers what that means for AM radio.

Why do high frequencies carry more data?

What you see: A phone showing "5G UW" or "5G+" and very fast downloads outdoors, which slow sharply once you step inside.

The answer is mostly arithmetic. The whole MF band, 300-3,000 kHz, is 2.7 MHz wide.⁶ The single block of C-band spectrum the FCC auctioned for 5G in 2021, 3.7-3.98 GHz, is 280 MHz wide by itself - about a hundred times the entire MF band.² The SHF band as a whole, 3-30 GHz, spans 27,000 MHz.⁶

A wider channel can carry more bits per second, the same way a wider highway carries more cars. Higher up the spectrum, there is room for wide channels; lower down, there is not. The price of that room is reach: the higher you go, the more easily walls, windows, leaves and human bodies absorb or reflect the signal. Networks handle this by stacking several bands at once and handing your phone between them, as the cellular section explains.

Why does my phone lose signal inside buildings?

What you see: Full bars on the sidewalk, one bar in the lobby, nothing in the elevator.

Every wall, floor and window between you and the cell site costs signal, and the cost climbs with frequency. Engineers measure it in decibels (dB); every 10 dB is a factor of ten in power. One industry reference summary gives these typical building penetration losses:⁹

At 28 GHz, the same summary calls concrete "effectively opaque."⁹ Treat these as ballpark figures: the page has internal inconsistencies and itself tells engineers to check the ITU's model (ITU-R P.2109) and the 3GPP 5G channel model (TR 38.901).⁹

The material matters as much as the frequency. Low-E glass - energy-saving window glass with a thin metallic coating that keeps heat in - partly reflects radio waves, and one industry summary likens the metallized coating to a Faraday cage.⁹ That summary puts its loss at about 25-35 dB at 3.5 GHz, against 5-8 dB for ordinary glass at 2 GHz.⁹ A glass office tower built to save energy can be harder to get a signal into than an old brick building with plain windows. Indoor antenna systems and small cells mounted close to the street are two ways around the problem; for the gear on poles, see https://streetcode.cold.fyi/utility-poles/.

What is "building entry loss," and how do engineers model it?

What you see: A coverage map that shows "good" service at an address where your phone struggles indoors.

Building entry loss is the signal lost between the outside of a building and a point inside it. The ITU's model for it, Recommendation ITU-R P.2109, does not give one number: it gives a frequency-dependent median that rises with the logarithm of the frequency, with coefficients that depend on building type, plus a random term to reflect how much real buildings vary.⁹ It sorts buildings into "traditional" and "thermally efficient" types. The 3GPP model used in 5G simulations makes a similar split into "low-loss" buildings (older, large windows) and "high-loss" buildings (modern, energy-efficient).⁹

Real buildings do not always fit those two boxes. A January 2024 study by Christopher R. Anderson at NTIA's Institute for Telecommunication Sciences, the US government's radio research lab, measured building entry loss at 1.7 GHz in one historic and one modern building. It found that material type and density matter a great deal, and that the P.2109 "traditional" versus "thermally efficient" categories may not capture enough construction detail.¹⁰

Outside, the city adds its own complications. In a downtown street, a signal rarely arrives on a single straight path: it reflects off buildings and bends over rooftops, and several copies of it arrive at slightly different times. This is called multipath. Phones and GPS receivers have to sort those copies out; the GPS section covers what that does to your blue dot.

GPS shows the cost in numbers. The US government's GPS website says GPS-enabled smartphones are typically accurate to within a 4.9 m (16 ft) radius under open sky, but that accuracy worsens near buildings, bridges and trees, and it lists signals reflected off buildings or walls - multipath - among the common causes of poor positioning.¹¹

Who discovered radio waves, and did Marconi invent radio?

What you see: A plaque, a museum display or a trivia question crediting one person with "inventing radio."

The honest answer is that it took several people. JSTOR Daily credits Hertz with proving that electromagnetic waves exist, and Marconi (1874-1937) with turning them into a working communications system.³ Marconi moved from Italy to England in 1896 to find funding, and in 1899 he sent signals across the English Channel.³

Other countries tell the origin story with different names at the center, and those claims are argued over by historians to this day. This guide credits each person with what the sources say they did, rather than naming one "inventor."

In Russia, the name is Aleksandr Popov. IEEE Spectrum reports that he demonstrated his radio-wave detector in a presentation at St. Petersburg University on May 7, 1895 (he later adapted it as a lightning detector), a date later declared Radio Day and still observed in Russia; Popov never sought a patent, while Marconi filed for a British patent on June 2, 1896.¹²

In the US, supporters of Nikola Tesla point to the courts: a history page at Spain's Universitat Pompeu Fabra says that in 1943, a few months after Tesla's death, the US Supreme Court overturned Marconi's patent in Tesla's favor.¹³ The ruling itself was narrower. Decided on June 21, 1943, it struck down the broad claims of Marconi's US tuning patent because earlier work by Oliver Lodge, John Stone Stone and Tesla had anticipated it; it said nothing about Marconi's original patent and did not decide who invented radio.¹⁴

What happened at Signal Hill in 1901?

What you see: References to Signal Hill in St. John's, Newfoundland, or to Poldhu in Cornwall, as the birthplace of transatlantic radio.

On December 12, 1901, Marconi and his assistant George Kemp, at Signal Hill near St. John's, Newfoundland, listened for the Morse letter "S" (three dots) being sent from a transmitter at Poldhu, Cornwall, in England.¹⁵ They used a wire antenna about 500 ft (150 m) long held up by a kite, and an untuned receiver. Marconi's diary records "Sigs. at 12:30, 1:10 and 2:20."¹⁵ The IEEE recognizes the event as an engineering milestone, though its own page notes that some historians suggest he may have heard a high-frequency harmonic of the transmitter rather than its intended signal.¹⁵

Did the Titanic lead to radio laws?

What you see: The claim, in documentaries and museum captions, that the Titanic disaster created radio regulation.

It sped it up. The Radio Act of 1912 was passed after the Titanic sank in April 1912. It required ocean-going ships to keep a radio room staffed 24 hours a day, and it put the US Department of Commerce in charge of licensing radio stations, mostly maritime and amateur ones at the time.¹⁶ It was approved on August 13, 1912.¹⁷ For the first time, anyone transmitting by radio in the US needed a government license.¹⁸

Those three ideas - a license to transmit, a federal agency in charge, and rules written partly for safety at sea - are still the core of radio law over a century later.

Why did the US need new radio laws in 1927 and 1934?

What you see: Station licenses, FCC filings and broadcast rules that all refer back to "the public interest."

The 1912 Act was written before broadcasting existed, and it broke under the strain. In 1926, Zenith's Chicago station WJAZ broadcast on a frequency it had not been assigned. On April 16, 1926, federal Judge James H. Wilkerson ruled that the government lacked authority to set operating requirements such as frequencies, and in July 1926 the Acting Attorney General agreed the law was inadequate.¹⁷ With nobody able to keep stations on their channels, Congress started over.

Federal rules today still name the 1934 Act as the authority under which the FCC runs its half of the US frequency table; the other half, for federal government users, is covered in the rules section.¹⁹

In Boston

Boston, like the rest of the 50 states, sits in ITU Region 2, and the US Table of Frequency Allocations that governs every transmitter in the city is based on the Region 2 table.¹⁹ The physics in this section is the same in Boston as anywhere else.

What Boston offers is a good place to see it. The city mixes older masonry buildings with newer glass towers, and the building entry loss research above predicts that the second kind will often be harder for an outdoor cell site to reach - especially at the higher 5G frequencies. If your phone holds a signal in an older brick building and struggles in a sealed-glass office, that is consistent with the published loss figures, which are general rather than measured in Boston.

Cape Cod has its own place in the history: the National Park Service marks a Marconi wireless station site at Wellfleet, now part of Cape Cod National Seashore, as the location of America's first wireless station.²⁰ On January 18, 1903 the South Wellfleet station took part in the first public two-way wireless communication between Europe and America, carrying messages from President Theodore Roosevelt and King Edward VII in Morse code.²⁰ The Navy closed the station in 1917 for wartime security, it was scrapped in 1920, and most of the site has since eroded into the Atlantic.²⁰

Elsewhere in the world

The band names and the physics are the same everywhere; what differs is who applies them and how. Australia's ACMA, as quoted above, plans its spectrum by matching services to the frequencies whose propagation suits them.⁷ The ITU's building entry loss model is an international recommendation, used by national regulators in coverage and sharing studies, while local building codes and energy rules decide how much low-E glass is in the windows.⁹

Radio's early history was international from the start: an Italian inventor who had moved to England for funding sent a signal from Cornwall to Newfoundland.³¹⁵ National radio laws followed in each country. In the UK, Ofcom's radio spectrum functions come from the Wireless Telegraphy Act 2006;²¹ how it and other regulators divide the airwaves is the subject of the rules section.

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03Who owns the airwaves

The short version: Nobody owns the radio spectrum outright; governments manage it, and treaties coordinate it across borders. In the US, the FCC manages spectrum for businesses, state and local governments and the public, while NTIA manages it for federal agencies, and many bands are shared. Regulators first decide which kind of service may use a band (allocation), then give specific users permission to use specific frequencies (assignment), increasingly by auction: the C-band auction that closed in January 2021 drew over $80.9 billion in gross bids. In the US, your Wi-Fi router needs no license at all under FCC Part 15, on the condition that it causes no harmful interference and puts up with any it receives.

Who owns the radio spectrum?

What you see: News stories about governments "selling the airwaves," or a carrier "buying spectrum."

What is bought and sold is permission, not property. Spectrum rules work in two steps:

• Allocation decides what kind of service may use a band: broadcasting, mobile phones, radar, satellites, amateur radio and so on.
• Assignment gives a specific station or licensee permission to use a specific frequency within that allocation.

US rules require frequency assignments to follow the allocation table, with listed exceptions.²² The UK spells out the same two steps in law: Ofcom must publish a UK Plan for Frequency Authorisation showing which frequencies are "allocated, for what purposes, and which are available for assignment."²¹

The formal definitions come from the ITU Radio Regulations, which US rules repeat in 47 CFR 2.1. An allocation is the entry of a frequency band in the Table of Frequency Allocations for use by one or more radio services under specified conditions. An allotment is the entry of a channel in an agreed plan, adopted by a competent conference, for use by one or more administrations in identified countries or areas. An assignment is an authorization given by an administration for a radio station to use a frequency or channel under specified conditions.²³

An allocation can be split between services ranked by priority. In the US table, primary services are printed in capital letters and secondary services in normal type. A secondary station must not cause harmful interference to a primary station and cannot claim protection from interference caused by one.¹⁹

What is the difference between the FCC and NTIA?

What you see: Two agencies' names on the same spectrum news story, often disagreeing.

The US runs two allocation tables side by side. The Federal Table is administered by the National Telecommunications and Information Administration (NTIA), part of the Commerce Department, for federal government users such as the military and federal agencies. The non-Federal Table is administered by the Federal Communications Commission (FCC) under the Communications Act of 1934, for everyone else.¹⁹ Together they make up the US Table of Frequency Allocations, in which a band can be federal-only, non-federal-only or shared between the two.¹⁹

The split matters whenever the country wants to move spectrum from government use to commercial use. A dispute over the 3.1-3.45 GHz band is part of the story of why Congress let the FCC's auction authority lapse in 2023 (see below).²⁴

What is the US frequency allocation chart?

What you see: A poster covered in hundreds of colored blocks, often on the wall of a radio shop, a classroom or an engineer's office.

It is NTIA's wall chart of the US Table of Frequency Allocations, which is set out in 47 CFR 2.106 and described in 47 CFR 2.105.¹⁹ Each color stands for a type of service, and each block shows which services share a slice of the spectrum. It is the best single picture of who uses the air in the US.

As of October 2026, the current edition is dated September 2025 and uses data as of March 2025; NTIA publishes it as a free PDF on ntia.gov.²⁵

What is the ITU and what does it do?

What you see: "ITU" or "ITU-R" in footnotes of spectrum rules, or in the news when countries argue over satellite orbits.

Radio waves ignore borders, so countries coordinate through the International Telecommunication Union (ITU), a UN agency with 194 member governments and over 1,000 companies and organizations as members.¹ It maintains the Radio Regulations, which the ITU calls "the international treaty" governing the radio spectrum and both geostationary and non-geostationary satellite orbits.²⁶ The 2024 edition is published online in all six UN languages.¹ The orbits themselves are covered in satellites.

The Radio Regulations divide the world into regions with their own allocation tables. The US table is based on the ITU Region 2 table, which covers the 50 states, the District of Columbia, Puerto Rico and the US Virgin Islands; Guam and American Samoa fall in Region 3.¹⁹ Each country fits its national rules inside this framework. Australia, for example, follows the ITU Radio Regulations and must avoid harmful interference to other countries' services that comply with them.⁷

US rules restate the ITU's three regions in 47 CFR 2.104, using three lines that run from pole to pole. Line B runs down the Atlantic (along 10°W and later 20°W), line C runs through the Bering Strait and across the Pacific (ending along 120°W), and line A runs along 40°E and then 60°E. Region 2, between lines B and C, is the Americas; Region 1, between lines A and B, takes in Europe and Africa and also, by name, Russia, Turkey, Ukraine and several Caucasus and Central Asian countries; Region 3, between lines C and A, covers the rest.²⁷ The ITU describes Region 3 as "the rest of Asia and most of Oceania."²⁸

When is the next World Radiocommunication Conference?

What you see: Headlines about "WRC-23" or "WRC-27," usually tied to 5G, 6G or satellite internet.

A World Radiocommunication Conference (WRC) is where the treaty gets rewritten. WRCs meet every three to four years to review and revise the Radio Regulations, on an agenda set by the ITU Council.²⁶

As of October 2026, WRC-27 is a year away. A WRC decision does not reach your phone directly: each country then has to write the change into its own rules.

Why does the government auction airwaves?

What you see: A carrier announcing it has "won" licenses in an FCC auction, with a price in the billions.

Congress first gave it authority to auction licenses in the Omnibus Budget Reconciliation Act of 1993 (Public Law 103-66, section 6002), originally set to expire on September 30, 1998.²⁹ The argument for auctions is that a bidding price, rather than an administrative decision, sorts out who gets a license; the government also collects the proceeds, as the budget figures below show.

The first auction, for ten nationwide narrowband PCS licenses, ran in July 1994. It took five days, used a simultaneous multiple-round ascending format, and raised about $617 million.³⁰ Auctions have grown since:

These are reported trade-press figures; the FCC's own auction pages give the official net and gross totals.

Why did the FCC lose its auction authority?

What you see: Stories from 2023-2025 about the FCC being unable to hold spectrum auctions.

Congress renewed the FCC's auction authority several times after 1993, but on March 9, 2023 it lapsed.²⁹ It was the first lapse since 1993, and it came amid a dispute over the 3.1-3.45 GHz band; 2024 was the first full year the FCC had no auction authority at all.²⁴ The lapse even stranded licenses T-Mobile had already won in Auction 108, until the 5G SALE Act required the FCC to issue them by March 18, 2024.²⁴

Authority was restored by Public Law 119-21, the FY2025 budget reconciliation law known as the "One Big Beautiful Bill Act," signed on July 4, 2025. It now runs through September 30, 2034.²⁹³¹ The law also set a pipeline of at least 800 MHz of spectrum to be sold:³¹

Sources disagree on the details. Light Reading's summary of the law gave different figures (600 MHz, a range of 1.3-10 GHz and 3- and 6-year deadlines); the Congressional Research Service report and the Wiley law firm's analysis agree with the version in the table.²⁴²⁹³¹ The Congressional Budget Office projected more than $85 billion in offsetting receipts from the reauthorization over FY2025-FY2034.²⁹ Separately, an auction of the remaining AWS-3 spectrum, required by the FY2025 defense authorization act (Public Law 118-159), was scheduled to start on June 2, 2026.²⁹

That auction, Auction 113, closed on June 23, 2026 with $3,572,889,200 in gross bids for 200 licenses, won by seven bidders, according to the FCC's closing notice of June 26, 2026.³² On July 22, 2026 the FCC adopted rules for an auction of 160 MHz of upper C-band spectrum at 3.98-4.14 GHz; Light Reading reported it was on track to close by the July 2027 statutory deadline, without a confirmed start date.³³

What does "FCC Part 15" on my device mean?

What you see: A label or settings screen reading "This device complies with Part 15 of the FCC Rules," usually beside an FCC ID.

It means the device is one of the low-power gadgets anyone can use without a radio license. Part 15 of the FCC's rules lets manufacturers build such devices; the FCC's rules for low-power unlicensed operation date back to November 1938, and since June 2000 private Telecommunication Certification Bodies have been able to certify devices on the FCC's behalf.¹⁶

The trade-off is written into the rule. Under 47 CFR 15.5, a Part 15 device:³⁴

• has no vested or recognizable right to any frequency;
• may operate only on the condition that it causes no harmful interference;
• must accept interference from licensed stations, from other devices and from industrial, scientific and medical (ISM) equipment;
• must stop operating if the FCC notifies the operator that it is causing harmful interference.

In practice, that is why, under US rules, your Wi-Fi has no recourse when a neighbor's network or a microwave oven crowds it: the rules never promised it a clear channel. The unlicensed section covers what lives in these bands.

What are ISM bands, and why do so many gadgets share them?

What you see: Wi-Fi that slows down when the microwave oven runs, or a list of "2.4 GHz" and "5 GHz" networks in your phone's settings.

ISM bands are slices of spectrum set aside for industrial, scientific and medical equipment that uses radio energy for something other than communication, such as heating food. Unlicensed radios share some of them. The US rule for frequency-hopping and digitally modulated unlicensed systems, 47 CFR 15.247, covers three bands:³⁵

Frequency hopping and digital modulation are the technology families that Bluetooth and Wi-Fi belong to, which is why these bands are crowded with consumer gear. Because Part 15 devices must accept interference from ISM equipment, when ISM equipment such as a microwave oven drowns out your Wi-Fi, the rules side with the oven.³⁴

The formal US list of ISM frequencies, in 47 CFR 18.301, has eleven center frequencies, each with a set tolerance: 6.78, 13.56, 27.12 and 40.68 MHz; 915 MHz (plus or minus 13 MHz); 2450 MHz (plus or minus 50 MHz); 5800 MHz (plus or minus 75 MHz); and 24.125, 61.25, 122.50 and 245.00 GHz.³⁶ Wi-Fi in the 5 GHz and 6 GHz ranges also runs under a separate rule: 47 CFR 15.403 defines Unlicensed National Information Infrastructure (U-NII) devices as radios using wideband digital modulation in 5.15-5.35 GHz, 5.47-5.895 GHz and 5.925-7.125 GHz.³⁷

What is 6 GHz Wi-Fi, and is it legal?

What you see: A router or phone advertising "Wi-Fi 6E" or "Wi-Fi 7," or a network name ending in "6G" in your list.

Yes, in the US it is legal and unlicensed. On April 23, 2020 the FCC opened 1,200 MHz of spectrum at 5.925-7.125 GHz for unlicensed use.³⁸ It came with two tiers:³⁸

The band is not empty: it is used by existing licensed services, which the rules protect. An AFC system is a database that tells each standard-power access point which channels it can use at its location without interfering with those incumbents.³⁸ Low-power indoor units do not use AFC; keeping them indoors and at low power limits how far their signals travel, thanks to the same building entry loss described in Radio in plain words.

In Boston

Every transmitter in Boston works under the arrangement described above. The city is in ITU Region 2, so the US table built on the Region 2 table applies; the FCC licenses the city's commercial, public safety and broadcast users, and NTIA handles federal government users.¹⁹ A Boston household's Wi-Fi router and other unlicensed gadgets run under the same Part 15 conditions as anywhere else in the country.³⁴

For how city agencies use licensed radio for 911 and dispatch, see https://works.cold.fyi/911/.

Elsewhere in the world

Every country manages its own spectrum inside the ITU framework, through its own regulator and allocation table.

The US split between a federal manager (NTIA) and a civil one (FCC) is written into its own rules.¹⁹ 6 GHz Wi-Fi is not the same everywhere. The US opened the whole 5.925-7.125 GHz band in 2020,³⁸ but other regions have taken different approaches.

In the UK, Wi-Fi already uses 5925-6425 MHz. In a statement of July 20, 2026, Ofcom decided to split the upper band: 6425-6585 MHz becomes a Wi-Fi priority portion for license-exempt use, including low-power indoor use, while in 6585-7125 MHz mobile has priority and Wi-Fi is allowed only under AFC control. The regulations to extend Wi-Fi into 6425-6585 MHz were still in draft, out for consultation, at the time of that statement.⁴¹

Canada's ISED opened the whole 5925-7125 MHz band to license-exempt use in May 2021, according to an industry summary of its decision.⁴² At WRC-23, countries agreed that those wishing to use the band for mobile (IMT) may consider 6425-7125 MHz in Region 1 or 7025-7125 MHz in Region 3, with protection for existing services such as satellite, and kept room for national choices between Wi-Fi and mobile.²⁸

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04AM, FM and TV: the broadcast bands

The short version: Broadcast radio and TV are the oldest residents of the airwaves over a city: one big transmitter, a tall tower, and any number of receivers that never talk back. AM radio carries sound by changing the strength of its signal and FM by changing its frequency; FM channels are also far wider, which is a big part of why FM sounds better. US TV went digital, then in 2016-2017 stations sold part of their band to wireless carriers and almost 1,000 stations moved channels, which is why so many people had to rescan their TVs.⁴³⁴⁴ As of October 2026 the next US standard, ATSC 3.0 or "NextGen TV," is still optional, and the FCC's plan to let stations drop the old signal was still a proposal when broadcasters filed comments on it in January 2026.⁴⁵⁴⁶

What is the difference between AM and FM?

What you see: Two bands on a car radio, one with numbers like 680 or 1030, the other with numbers like 89.7 or 102.5.

AM (amplitude modulation) sends sound by varying the strength of a steady radio carrier. FM (frequency modulation) keeps the strength steady and nudges the frequency up and down instead. In the US, AM carrier frequencies fall between 535 and 1605 kHz, and stations are assigned frequencies from 540 to 1600 kHz in 10 kHz steps.⁴⁷

FCC rules define the US AM broadcast band as 535-1705 kHz, with 117 carrier frequencies running from 540 kHz to 1700 kHz in 10 kHz steps.⁴⁸ For the 1605-1705 kHz segment, the rules describe a model station with full-time stereo operation, 10 kW of power by day and 1 kW at night, spaced 400-800 km from other stations on the same channel.⁴⁸

FM lives far higher, at 88 to 108 MHz, wedged between VHF TV channels 6 and 7.⁴⁷ Its channels are 200 kHz apart, with centers starting at 88.1 MHz, which leaves room for up to 100 stations.⁴⁷ That is why US FM frequencies always end in an odd tenth: 88.1, 88.3, 88.5 and so on. An FM station's frequency swings by up to 75 kHz either side of center, and 25 kHz guard bands keep neighbors apart.⁴⁷ A 200 kHz FM channel is 20 times wider than a 10 kHz AM slot, and that extra room is what lets FM carry higher-fidelity sound.

The two bands also behave differently over distance. AM's long medium-wave signals follow the curve of the ground and, after dark, can bounce off the upper atmosphere and land hundreds of miles (several hundred km) away, which is why distant AM stations pop up on the dial at night. FM at 100 MHz behaves more like light: it mostly reaches as far as the antenna can "see," plus a little.

How does FM carry stereo on one station?

What you see: A "stereo" light that comes on when the signal is strong and flickers off on the edge of a station's range.

FM stereo is a clever bit of backward compatibility. The main signal carries left plus right (L+R), which is all a mono radio needs. A 38 kHz subcarrier, tucked inside the same channel, carries left minus right (L-R).⁴⁷ A stereo receiver adds and subtracts the two to rebuild separate left and right channels. The L-R part is weaker and noisier, so when the signal fades many radios drop to mono to keep the sound clean, which is the light you see going out.

What is HD Radio, and what is DAB?

What you see: An "HD" logo on a car stereo, extra channels labeled "HD2" or "HD3," or in the UK and Europe, a radio marked "DAB" or "DAB+."

Countries took two different roads to digital radio.

The US chose HD Radio, an in-band on-channel (IBOC) system: digital sidebands ride alongside a station's existing AM or FM signal, so the station keeps its familiar spot on the dial and old radios keep working. The approval process began with a 1998 petition from USA Digital Radio, and the FCC approved IBOC as the US method on October 11, 2002.⁴⁹ The FCC adopted a further proposal on detailed HD Radio rules on April 20, 2004.⁴⁹

The UK and much of Europe chose DAB and its newer version DAB+. Instead of each station sitting on its own frequency, many stations are bundled into one transmitted block called a multiplex.

In the UK in the second quarter of 2026, 77% of the population listened to digital radio each week, and DAB accounted for 57% of digital listening hours, ahead of smart speakers (25%), web and apps (15%) and digital TV (3%).⁵⁰ In 2021 figures, the UK had 495 DAB and 56 DAB+ programs on air; 75 small-scale DAB multiplexes had launched by September 2025, with about 100 or more planned.⁵⁰ In 2021 figures, DAB covered 98% of the UK for national BBC stations, 91.5% for national commercial stations and 91% for local stations.⁵⁰

The BBC started DAB engineering test transmissions in 1990 and launched the service to the public in September 1995.⁵¹ UK multiplexes broadcast on frequency blocks in VHF Band III, with block center frequencies from 174.928 MHz (block 5A) to 229.072 MHz (block 12D).⁵¹

Why did TV go digital, and why did I have to rescan my TV?

What you see: A TV that lost channels until you ran "channel scan" again, or a channel number on screen (like 2.1) that no longer matches the frequency the station actually uses.

US full-power stations switched from analog to the digital standard ATSC 1.0, which is still what most over-the-air TVs receive.

The FCC says that since June 12, 2009, all full-power television stations in the US have stopped broadcasting in analog.⁵²

The second upheaval, and the reason for the rescans in 2019 and 2020, was the repack. After the incentive auction (next item), the FCC moved stations to new channels so the freed spectrum could go to cellular. The repack ran over 39 months in ten phases, scheduled to finish in July 2020.⁴³ In all, 987 full-power stations were either repacked or among six holdouts given extensions into August and September 2020, and the repack was estimated to cost about $2.75 billion.⁴⁴ The FCC's chairman said in October 2020 that the 600 MHz repack had been completed that summer.⁵³

Digital TV keeps a "virtual" channel number for branding, so a station that physically moved could keep showing viewers the same number. Your TV only learns the new physical frequency by scanning, which is why a rescan brought channels back.

What was the FCC incentive auction?

What you see: Nothing on the street. You see the result in your phone's coverage: the 600 MHz band that now carries cellular service used to be TV channels.⁵³

The incentive auction was a two-sided sale. In the reverse auction, TV stations offered to give up spectrum for money; in the forward auction, wireless carriers bid to buy it. Bidding closed on March 30, 2017, and the FCC announced the close on April 13, 2017.⁴³ Carriers' gross bids came to $19.8 billion, the reverse auction paid $10.05 billion to 175 stations, and more than $7.3 billion went to the US Treasury.⁴³ Forty-one stations took the money and went off the air entirely.⁴⁴

The auction repurposed 84 MHz of former TV spectrum: 70 MHz for licensed wireless service and 14 MHz for wireless microphones and unlicensed use.⁴³ That last slice matters to anyone running a stage or a church PA; the wireless mic side of the story is in show wireless.

What is NextGen TV (ATSC 3.0), and is regular TV being shut off?

What you see: A "NextGen TV" logo on a new set or converter box, or news stories saying broadcasters want to switch off the old signal.

ATSC 3.0, marketed as NextGen TV, is a newer broadcast standard built on internet protocol. Stations may adopt it voluntarily, but as of the FCC's October 2025 notice they still had to keep an ATSC 1.0 simulcast running so existing TVs still work. A 2023 FCC order (June 23, 2023) let 3.0 stations contract with other stations to host their 1.0 simulcast.⁵⁴

On October 28, 2025, the FCC adopted a Fifth Further Notice of Proposed Rulemaking (FCC 25-72, GN Docket 16-142), released the next day. It tentatively concluded that mandatory 1.0 simulcasting should end while voluntary simulcasting would remain, and proposed removing the rule that a station's 1.0 simulcast be "substantially similar" to its 3.0 programming ahead of that rule's scheduled 2027 sunset.⁴⁵ It also asked whether new TVs should be required to have 3.0 tuners, and asked about encryption of over-the-air signals.⁴⁵ Encryption (digital rights management) is contested: some viewers and device makers object to it, and the FCC sought comment rather than deciding.⁴⁵

The National Association of Broadcasters had petitioned in February 2025 for hard deadlines: a full switch by February 2028 in the top 55 markets and February 2030 everywhere else, with no deadline proposed for low-power stations and translators.⁴⁵ The FCC's notice did not adopt those dates, and in comments filed on January 20, 2026, the NAB again asked for a firm 1.0 sunset date.⁴⁶ The ATSC, the standards body, said the October 2025 vote was unanimous and called it "the beginning of a longer process."⁵⁵

How far 3.0 has spread depends on who is counting. The FCC's notice says 3.0 service has launched in more than 90 markets counting all station classes (more than 80 for full-power stations alone), covering more than 70% of the population, but only seven new markets since January 2024; more than 14 million 3.0-capable sets and 300,000 converters had sold through 2024, and no cable or satellite provider carried 3.0.⁴⁵ Cord Cutters News reported about 75% of US households could receive NextGen signals in 60-plus markets.⁵⁶

In Boston: what are the towers in Needham?

What you see: Driving Route 128 through Needham, a cluster of very tall lit towers, one with a wide triangular top holding several antennas.

Much of Boston's over-the-air TV comes from two tower sites in Needham, just outside the city, which back each other up.⁵⁷ For how to read tall structures like these, see towers.

The Cedar Street tower was built in 1957 for WBZ-TV. WGBH-TV, WCVB and WGBX joined it later, and it was rebuilt taller during the 2000s digital conversion, adding WSBK.⁵⁷ WGBH-TV moved its transmitter to Needham on June 18, 1966, and its signal is rated at a height above average terrain (HAAT) of 362.7 m (1,190 ft).⁵⁸ HAAT compares the antenna's height with the average ground level around it, which is a better guide to coverage than the tower's own height.

The Cabot Street candelabra, about a mile (1.6 km) southeast across Route 128, went up at the end of the 1960s. A candelabra tower has a wide top frame that holds several stations' antennas side by side. This one originally carried WFXT (channel 25), WSBK (38) and WLVI (56).⁵⁷ It was rebuilt with a UHF antenna on top and a large low-band VHF antenna for WGBH, finished in 2019, and is now the main site for WGBH and WFXT and the full-power backup for WBZ-TV, WCVB, WSBK and WGBX.⁵⁷

The repack reshuffled things here too. WGBH moved down to low-band VHF channel 5, and WLVI gave up its UHF spectrum and now shares a channel with WHDH.⁵⁷

The Cabot Street candelabra also became home to a backup facility for several of the market's big FM stations, and WSBK's old building there became, at least in part, the FM auxiliary site.⁵⁷ The structural database Structurae lists the Needham UHF candelabra, completed in 1968, at 366 m (about 1,200 ft).⁵⁹

Elsewhere in the world

What you see: A different set of logos on radios and TVs abroad, or a UK radio marked DAB.

• Digital radio: the US uses HD Radio, approved by the FCC in 2002.⁴⁹ The UK went the DAB route, and by Q2 2026 digital radio reached 77% of the UK population each week.⁵⁰
• Digital TV: the US repack that freed the 600 MHz band for wireless finished in 2020.⁵³
• AM spacing: most medium-wave stations outside North and South America operate on exact multiples of 9 kHz rather than the 10 kHz used in the US, under the Geneva frequency plan of 1975, which entered into force on November 23, 1978.⁶⁰
• FM in Japan: the band runs 76-95 MHz, extended from 76-90 MHz after Japan's analog TV shutdown on July 24, 2011.⁶¹
• Digital TV standards: Europe mainly uses DVB-T and DVB-T2; Japan uses ISDB-T.⁶²

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05How your phone talks to the network

The short version: A cell network divides an area into many small zones, each served by a low-power transmitter, reuses the same frequencies in zones that are far enough apart, and hands your call from one zone to the next as you move. That idea launched commercially in the US in Chicago on October 13, 1983, and every generation since has built on it.⁶³ Today the signal reaches you from big macro towers, from small cells on poles, and in places like subway tunnels from a distributed antenna system. US 5G runs on low, mid and very high (mmWave) bands, and the C-band slice of the mid band set off a long dispute with aviation over aircraft radio altimeters.⁵³⁶⁴

How does a cell network work?

What you see: Bars on your phone that rise and fall as you walk, and calls that keep going as you drive across town.

The founding trick of cellular is reuse. When Ameritech Mobile Communications opened the first US commercial cellular service in Chicago on October 13, 1983, it used low-power transmitters that each covered part of the city, reused the same frequencies in different areas, and had computers hand calls off from one area to the next as a caller moved.⁶³ Phones then cost about $2,200 to $3,500, with monthly charges of $150 to $200.⁶³

A cell site is the set of antennas, radios and backhaul links that serves one coverage area. A handoff (or handover) is the moment the network passes your connection from one site to another without dropping it. Because each site is low power, a frequency used in one neighborhood can be used again a few cells away, which is how a network serves millions of phones with a limited slice of spectrum.

Under the 3GPP standards published by ETSI, a phone that is switched on first selects a network (a PLMN, in 3GPP terms), then searches for a suitable cell on that network and "camps" on it, monitoring its control channel.⁶⁵ It may then register its presence in the tracking area of that cell, and if it later finds a more suitable cell under the reselection criteria, it moves there, using its own signal measurements to decide.⁶⁵ Those rules, from the 5G specification ETSI TS 138 304, cover a phone sitting idle; the handoff described above applies during a call or data session.⁶⁵

What do 1G, 2G, 3G, 4G and 5G mean?

What you see: A "5G," "LTE" or "4G" icon next to your signal bars, or an old phone that stopped working.

"G" stands for generation. Each generation changed how the radio link encodes voice and data, while keeping the reuse-and-handoff idea that the 1983 Chicago system used.⁶³ By 2024, US wireless connections numbered 579 million, about half of them 5G devices, according to the wireless trade group CTIA's annual survey.⁶⁶

Older generations do not last forever. Carriers switch them off to reuse the spectrum for newer service, which strands phones, alarm panels and car systems that only speak the old language (see "Why did my old phone stop working?" below).

In the US, Verizon switched on its LTE network on December 5, 2010, covering 38 metropolitan areas and more than 60 airports at launch.⁶⁷ Verizon launched mobile 5G on April 3, 2019, starting in parts of Chicago and Minneapolis.⁶⁸

Is that a cell tower or a small cell?

What you see: Either a tall lattice or monopole tower with rows of rectangular panel antennas near the top, or a slim canister or box on a light pole or building corner with a power meter and cabinet nearby.

A macro site is the classic cell tower or rooftop installation: high up, relatively powerful, covering a wide area. A small cell is a low-power site mounted on a pole or building, covering a much smaller area, added mainly to give a busy area more capacity rather than more reach. Small cells have spread fast: tower company Crown Castle alone counted about 115,000 small cells on air or under contract in 2022.⁶⁹

The two work together. The macro layer gives everyone a signal; small cells take the load in crowded blocks, so the macro site is not trying to serve a whole downtown at once. The street furniture that carries small cells, from poles to the cabinets bolted beside them, is covered in detail in the utility poles guide.

How many cell towers are there in the US?

What you see: A tower on a hill, a cluster of panels on an apartment roof, a church steeple that seems to have grown extra hardware.

Counts depend on what you call a site; towers explains how to tell the structures apart. CTIA counted 418,887 operational US cell sites at the end of 2021, a figure that does not include new 5G base stations added at existing sites.⁶⁹ CTIA's head cited more than 15,000 new cell towers activated in 2024.⁷⁰

The demand behind those numbers: Americans used about 132 trillion megabytes of mobile data in 2024, up from 100 trillion the year before.⁶⁶

What do low-band, mid-band and mmWave 5G mean?

What you see: Coverage maps with different shades for different kinds of 5G, and phones that are dramatically faster on some street corners than others.

US 5G runs on three tiers of spectrum. Lower frequencies travel farther and get through walls better; higher frequencies carry more data but fade quickly.

The FCC's "5G FAST" program auctioned the 24 and 28 GHz mmWave bands in 2019, and an auction of the upper 37, 39 and 47 GHz bands concluded in 2020, releasing 3,400 MHz in all.⁵³ For the mid band, the FCC auctioned 70 MHz of licensed 3.5 GHz (CBRS) spectrum in August 2020 and planned to clear the lower 300 MHz of the 3.7-4.2 GHz satellite band (the C-band) and auction 280 MHz of it, together creating a contiguous 530 MHz swath from 3.45 to 3.98 GHz.⁵³

The C-band sale, Auction 107, offered 280 MHz at 3.7-3.98 GHz in 5,684 blocks. Its clock phase ended in January 2021 at $80.9 billion gross, the highest-grossing US spectrum auction.⁷¹ For comparison, the forward auction for the 600 MHz band had raised $19.8 billion in 2017.⁴³

Why did 5G cause problems for airplanes?

What you see: News stories in late 2021 and early 2022 about canceled flights and "5G buffer zones" around airports.

A radio altimeter measures an aircraft's height above the ground by bouncing radio signals off it, and it matters most during low-visibility landings. These instruments work in a frequency band close to the top of the C-band, which is how the two came into conflict. The aviation section covers the rest of a plane's radios.

The FAA's December 2021 airworthiness directive states that commercial aviation radio altimeters operate in the 4.2-4.4 GHz band, while US 5G C-band service operates at 3.7-3.98 GHz, 220 MHz below.⁷²

In December 2021 the FAA found that 5G C-band interference could stop radio altimeters from working properly and issued an airworthiness directive.⁶⁴ In January 2022 Verizon and AT&T voluntarily added mitigations around airports.⁶⁴ In March 2023, Verizon, AT&T, T-Mobile and UScellular agreed in writing to keep those mitigations until January 1, 2028.⁶⁴

The FAA then set deadlines for the aircraft side. From June 30, 2023, planes without approved altimeters were barred from certain low-visibility operations, and from February 1, 2024, US airliners flying under Part 121 had to have approved altimeters.⁶⁴ About 180 planes needed new altimeters and 820 needed filters, at an estimated cost of $35.2 million.⁶⁴

The fix, in short, came from both ends: carriers kept mitigations in place around airports for a period of years, and airlines replaced or filtered the older altimeters that were most exposed.

What is a DAS, and why is there signal in some tunnels?

What you see: Small white antennas or flat panels on tunnel walls or station ceilings, and coaxial cable running along the tunnel.

A distributed antenna system (DAS) is a network of small antennas fed from a central hub, called a head end, used where signals from outside towers cannot reach: subway tunnels, stadiums, big buildings. Each carrier connects its radios at the head end, and the DAS carries the signal out along cable to antennas spread through the space. One shared system can then serve several carriers, instead of each company wiring the same tunnel on its own.

In Boston

What you see: Bars that hold steady as a train runs underground between stations on the T.

The MBTA signed a 15-year contract with InSite Wireless in 2005 to wire its tunnels for cellular service.⁷³ Coverage arrived in pieces: Verizon served four downtown stations from 2007, AT&T and T-Mobile had the Orange Line and part of the Red Line from 2010, and T-Mobile added the Blue Line and most of the Green Line in March 2012, with full wiring targeted for the end of 2012.⁷³

Later, American Tower worked with the MBTA to replace the original 2005 DAS equipment and add 5G.⁷⁴

The sources describe the tunnel network differently. The Boston Globe in 2012 put it at 19 miles of tunnel and 35 underground stations;⁷³ the Wireless Infrastructure Association's 2026 case study says more than 20 miles of tunnels linking more than 40 stations.⁷⁴ The difference may reflect what each counts as a tunnel or a station, or later expansion; neither source explains it.

An October 2016 article by InSite Wireless's DAS technology chief said five companies were connected to the T's tunnel DAS - AT&T, Sprint, T-Mobile and Verizon, plus Comcast for Wi-Fi on station platforms - after a build in five phases between 2007 and 2015.⁷⁵ The WIA case study gives no completion date or carrier list for the 5G upgrade.⁷⁴

Why did my old phone stop working?

What you see: An older phone, smartwatch, home alarm or car system that lost service in 2022.

US carriers scheduled their 3G shutdowns for 2022 and early 2023. As of February 2022, AT&T planned to switch off 3G on February 22, 2022, with about 2.7 million customers possibly affected.⁷⁶ T-Mobile set March 31, 2022 for its 3G CDMA network (inherited from Sprint) and July 1, 2022 for its own 3G UMTS network.⁷⁶ Verizon, which had originally targeted the end of 2019, set January 1, 2023.⁷⁶

A device that only speaks 3G has nothing to connect to once that happens, no matter how strong the bars look on a newer phone beside it.

Elsewhere in the world

What you see: Different 5G band numbers on a phone's spec sheet, or a UK phone that lost 3G between 2023 and 2025.

UK 5G spectrum. Ofcom, the UK regulator, consulted on its 700 MHz and 3.6-3.8 GHz auction in December 2018 and published principal-stage results on March 17, 2021.⁷⁷ Final results on April 27, 2021 showed £1,379,400,000 raised in total. EE won 3680-3720 MHz plus 700 MHz blocks; Telefonica (O2) won 3760-3800 MHz plus 700 MHz; Vodafone won 3720-3760 MHz; and Three won paired 700 MHz spectrum.⁷⁸ The UK's assigned 3680-3800 MHz overlaps the bottom of the US C-band (3.7-3.98 GHz).⁷¹

UK 3G and 2G switch-off. As of February 2023, Ofcom reported that Vodafone's 3G switch-off was starting early that year in Plymouth and Basingstoke, EE would start in early 2024, Three would finish by the end of 2024, and all operators had committed not to offer 2G or 3G service beyond 2033.⁷⁹ Virgin Media O2 was the last, with Durham the first city to lose its 3G, on April 2, and the full switch-off due by the end of 2025, by which time Vodafone, EE and Three had finished.⁸⁰ In other words, UK operators planned for 2G to outlive 3G, with 2033 as the outer limit.⁷⁹

In a post dated December 18, 2025, Virgin Media O2's chief technology officer said the company would complete its 3G switch-off "in the coming weeks," and that some customers might still see a 3G signal for a short period in early 2026.⁸¹

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06Police, fire and the radios that dispatch them

The short version: Police officers, firefighters, paramedics, railroads and utilities talk on land mobile radio, a push-to-talk system that runs alongside cell phones rather than through them. In the US, many large public safety systems use the digital P25 standard, and first responders also have a dedicated broadband network, FirstNet, on 700 MHz. Greater Boston adds a regional twist: its police mutual aid network, BAPERN, lives on T-Band channels that Congress once ordered sold off and then saved. The same airwaves carry warnings to the public, from the weather radio on 162 MHz to the alert that makes every phone in a room go off at once.

What is the radio on an officer's shoulder?

What you see: A handheld radio on a vest, a speaker-microphone clipped near the collar, and a bigger radio mounted in the cruiser or fire truck.

That is land mobile radio (LMR), the push-to-talk radio of police, fire, EMS, utilities, transit and railroads. CISA, the federal cybersecurity and infrastructure agency, describes a typical LMR system as handheld portables, mobile radios in vehicles, base stations, a network tying them together, and repeaters⁸².

A repeater is a radio at a high site that receives what a handheld sends and retransmits it. In CISA's words, "Repeaters are used to increase the effective communications range"⁸². Without a repeater, radios talk directly to each other on one frequency, usually called simplex. That works across a fire scene and fails across a city, because a handheld puts out little power from shoulder height.

US public safety LMR runs on VHF, UHF, 700 MHz and 800 MHz⁸². CISA warns that "Systems that operate on different frequencies are not always interoperable," so agencies use bridging devices to patch one system into another⁸². A town on VHF and a neighbor on 800 MHz cannot talk radio to radio without that kind of help.

There are two ways to organize the channels:

On a trunked system, users join a talkgroup, a virtual channel that the computer maps onto whichever real frequency is free for each conversation.

What is P25, and what does "Phase 2" mean?

What you see: A department announcing a new "digital radio system," or a scanner listing that says "P25 Phase 1" or "P25 Phase 2."

P25 (Project 25) is the North American standard for digital public safety radio. It was established in October 1989 by APCO, NASTD and three federal bodies: the NCS, NTIA and NSA⁸⁴. Its central promise is the Common Air Interface, which lets radios and infrastructure work together regardless of who made them; the Department of Homeland Security runs a Compliance Assessment Program to test that claim⁸⁴. Before standards like this, a department that bought one vendor's radios could find itself locked into that vendor.

P25 comes in two phases:

FDMA divides spectrum into separate frequencies; TDMA divides one frequency into separate turns in time. Phase 2 meets the FCC's 6.25 kHz-equivalent efficiency target in VHF, UHF and 700 MHz⁸⁴, which in practice means two conversations where Phase 1 fit one. The Phase 2 standard was approved in November 2010, and Motorola began shipping Phase 2 systems in August 2011⁸³.

On a trunked P25 system, a control channel assigns the voice channels. On a conventional P25 system, each packet carries a 12-bit Network Access Code (NAC) in place of the analog CTCSS and DCS tones older radios used to keep groups apart⁸³.

The standard also "supports secure communication through the use of encryption, key management, and equipment authentication"⁸⁴. Encryption is an option an agency turns on, not a default of P25, and that option is the technical root of the debate covered in scanners. For how a 911 call becomes a dispatch on these radios, see the sister guide at https://works.cold.fyi/911/.

What is FirstNet?

What you see: A rugged phone, tablet or vehicle modem used by a first responder, carrying data rather than push-to-talk voice over the traditional radio.

FirstNet is the US nationwide broadband network for first responders. Congress created it in Title VI of the Middle Class Tax Relief and Job Creation Act of 2012, signed February 22, 2012, as an independent authority within the NTIA at the Commerce Department⁸⁵. Congress provided $7 billion from spectrum auction revenues⁸⁵.

It runs on Band 14: 20 MHz of paired 700 MHz spectrum, 758-768 MHz from tower to device and 788-798 MHz from device to tower, which combined existing public safety broadband spectrum with the 10 MHz "D Block"⁸⁵.

On March 30, 2017, FirstNet awarded AT&T a 25-year contract, with $6.5 billion in FirstNet funding over the first five years; AT&T committed $40 billion over the life of the contract and access to infrastructure valued at more than $180 billion⁸⁵. By January 2018 all 50 states and 6 territories had opted in, though opting in does not oblige any agency to subscribe⁸⁵.

FirstNet is a separate system from the LMR radios above; a department can use both.

In its fiscal 2025 annual report, covering October 2024 to September 2025, the FirstNet Authority reported more than 7.8 million device connections and about 30,600 subscribed public safety agencies and organizations.⁸⁶ The initial nationwide buildout finished in March 2023, and in 2023 the FCC renewed the Band 14 license for ten years, contingent on the FirstNet Authority being reauthorized by February 22, 2027, the date it otherwise sunsets.⁸⁶ The same report says the House passed H.R. 7386 in April 2026 to extend the authority through September 2037, but that it was not yet law.⁸⁶

In Boston: what are BAPERN and the T-Band?

What you see: A scanner listing for Greater Boston with entries like "BAPERN 3," "BAPERN Central" or "MetroFire," on frequencies in the 470 MHz range.

BAPERN, the Boston Area Police Emergency Radio Network, has been developed and maintained by the Greater Boston Police Council since the early 1970s⁸⁷. It has more than 120 full member agencies, wide-area channels BAPERN 3 and 4, and district channels for the North, North-West, West, Central and South areas⁸⁷. It is a simulcast system - several transmitter sites sending the same signal at once - with voting receivers linked to Boston Police Headquarters, and each radio sends a unique ID that is logged with the voice traffic⁸⁷. A voting receiver setup listens at several sites and picks the clearest copy of each transmission.

Most of it runs on the T-Band, 470-512 MHz⁸⁸. According to CISA:

Boston and Cambridge police earlier used 460 MHz⁸⁹. The 6 MHz blocks are numbered like the UHF television channels they share space with.

CISA counts more than 200 T-Band licensees in Metro Boston, and a common regional channel plan covering more than 6,000 police, fire and EMS radios⁸⁹. A 2007 waiver yielded 32 repeater pairs and 6 simplex channels for that plan⁸⁹. MetroFire, a group of 34 communities, uses T-Band for fire mutual aid, and CISA cites the April 2013 Marathon bombing response as showing the strength of the region's shared radio⁸⁹.

The near-miss. Section 6103 of the same 2012 law that created FirstNet required public safety T-Band to be auctioned⁸⁸. For Greater Boston, that meant planning to move thousands of radios. The Don't Break Up the T-Band Act, part of the Consolidated Appropriations Act, 2021, signed December 27, 2020, repealed the auction mandate⁸⁸.

The newer layer. A 700 MHz P25 simulcast trunked overlay was in service when CISA wrote its case study, with seven sites and 10 to 12 planned⁸⁹. Boston Police itself moved to an encrypted digital system on August 9, 2025⁹⁰; scanners covers what that means for listeners.

What is that chatter near the train tracks?

What you see: Short calls between a train crew and a dispatcher or a track crew, heard on a scanner near a rail line, often in plain analog FM.

US railroads have their own block of VHF channels. NTIA's engineering report puts the railroad band at 160.215-161.565 MHz, with an existing plan of "181 frequencies, spaced every 7.5 kHz," while older analog equipment used 15 kHz channels⁹¹. As of NTIA's 1999 report, an industry-government Wireless Communications Task Force was planning a migration to narrowband digital radio⁹¹.

Much of that traffic is still analog. RadioReference's listings for Arkansas, as one example, show conventional analog FM channels between about 160.025 and 161.550 MHz for BNSF, CPKC, Union Pacific and shortline railroads⁹². The same listings note that railroads plan to move some secondary operations to the digital NXDN format, which not every scanner decodes⁹².

Utilities and transit agencies run their own LMR systems too.

What is the loud tone before an emergency message on TV or radio?

What you see: A harsh two-tone alarm and a crawl or voice announcement interrupting a broadcast, or a scheduled "this is a test."

That is the Emergency Alert System (EAS). Its lineage starts with CONELRAD, created under President Truman in 1951; the Emergency Broadcast System replaced it in 1963 under President Kennedy, and the FCC replaced that with EAS between 1994 and 1997⁹³. EAS participants include broadcasters, cable, wireless cable, satellite radio and, since May 2007, direct broadcast satellite TV⁹³.

Three agencies share the job. The FCC sets the technical rules, FEMA handles national activation and testing, and the National Weather Service issues weather alerts⁹³. FEMA's IPAWS system distributes digital alerts to broadcast, computers and phones⁹³. The first nationwide EAS test ran on November 9, 2011, at 2:00 p.m. Eastern, and lasted about 30 seconds⁹³.

Why did every phone in the room go off at once?

What you see: A loud tone and vibration on every phone nearby, with a short message about weather, a missing child or a local emergency.

That is a Wireless Emergency Alert (WEA), launched in April 2012⁹⁴. Alerts go out by cell broadcast: one message sent to every phone connected to the chosen cell towers, not a text to a list of numbers. That is why the phones of strangers sitting beside you go off at the same moment.

FCC rule changes since launch, per the FCC⁹⁴:

What is on 162.475 MHz?

What you see: A small "weather radio" on a kitchen counter or boat that stays silent until it blares an alarm, or a robotic voice reading forecasts on a scanner.

NOAA Weather Radio is the National Weather Service's own broadcast network. It runs 24 hours a day on seven VHF channels from 162.400 to 162.550 MHz, from 1,035 stations as of April 2025; each station covers about 40 miles (about 65 km), and together they reach more than 95% of the US population⁹⁵.

The trick that makes a weather radio useful at 3 a.m. is SAME (Specific Area Message Encoding): digital codes that activate only the receivers set for a given county or marine area⁹⁵. The NWS calls SAME the primary activator for EAS, so a weather warning on NOAA Weather Radio is often what triggers the tones on local TV and radio⁹⁵. A January 1975 White House policy named the network "the sole Government-operated radio system" for warnings into homes⁹⁵.

In Boston, the NOAA Weather Radio signal is on 162.475 MHz from Blue Hill, Massachusetts, according to the Town of Weymouth⁹⁶.

Elsewhere in the world

What you see: A national alert on a phone in Europe or the UK, sometimes with a different name and sound from the US version.

European Union. Article 110 of the European Electronic Communications Code requires member states to send "geo-targeted emergency alerts to all mobile phones located in the affected area," with a deadline of June 2022⁹⁷. EENA, the European emergency number association, reported that many countries missed it and still lacked compliant systems as of September 2023⁹⁷. France's FR-Alert combines cell broadcast with location-based SMS⁹⁷.

United Kingdom. The UK's Emergency Alerts system had its national test at 3 p.m. on Sunday, April 23, 2023, after pilots in East Suffolk and Reading⁹⁸. It works on 4G and 5G phones by mobile broadcast, reaches nearly 90% of phones in an area, and sounds and vibrates for up to 10 seconds⁹⁸.

UK emergency services radio. UK emergency services use Airwave, a TETRA (Terrestrial Trunked Radio) network.⁹⁹¹⁰⁰ Its planned replacement, the Emergency Services Network (ESN), is run by the Emergency Services Mobile Communications Programme and is meant to carry voice, video and data over a 4G/5G network; BT/EE holds the mobile services contract and IBM the user services contract, both in effect since December 2024.¹⁰⁰

The government's summary business case of July 27, 2026 puts the whole-life budget at £19.1 billion to 2043/44, with £6.1 billion already spent, and gives no Airwave shutdown date, saying only that it will close "when it is safe to do so."¹⁰⁰ Ofcom says the national Airwave infrastructure provision is planned to expire on December 31, 2026, and that ESN will serve England, Scotland and Wales.¹⁰¹

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07Scanners, encryption and what you're allowed to hear

The short version: In the US, owning a scanner and listening to unencrypted public safety radio is generally legal under federal law, but sharing or profiting from what you hear can violate the Communications Act, scanners that can receive cell phone calls are banned, and a few states restrict scanners in vehicles. The UK is stricter: using a receiver with intent to learn the contents of a message not meant for you is an offense. Meanwhile, many US police departments, including Boston's since August 2025, have encrypted their radios, and the arguments for and against that move are still being made in statehouses and newsrooms.

Is it legal to own a police scanner in the US?

What you see: A handheld or desktop scanner for sale at an electronics shop, or a scanner app on a neighbor's phone playing dispatch audio.

A scanner is a receiver that steps through many frequencies, or follows a trunked system's control channel, to find active transmissions. (A software-defined radio on a computer can do the same job; see hobby radio.) Federally, the key rule is Section 705 of the Communications Act (47 U.S.C. 605). According to the FCC's consumer guide, Section 705 "generally does not prohibit the mere interception of radio communications," and monitoring scanner traffic such as emergency reports is not a violation¹⁰².

The other federal law in play is the Wiretap Act. It permits interception of radio communications that are "readily accessible to the general public" (18 U.S.C. 2511(2)(g)(ii))¹⁰³. Unencrypted public safety radio falls on the permitted side of that line; encrypted traffic does not, as covered below.

Federal permission is not the whole picture. Some states add their own rules, mostly about scanners in vehicles, and the FCC notes that intercepting cellular, cordless or landline calls may violate other federal law (18 U.S.C. 2510-2520) with severe criminal penalties¹⁰².

Can I post or use what I hear on a scanner?

What you see: A neighborhood social media page relaying "what's on the scanner," or a tow truck arriving at a crash suspiciously fast.

Listening and divulging are treated differently. The FCC's guide says "a violation of Section 705 would occur if you divulge or publish what you hear" or use it for your own benefit¹⁰². Its example is a tow-truck operator who hears about an accident on police radio and dispatches a truck to get the business¹⁰².

The divulgence ban does not apply to broadcasts meant for general public use, to distress traffic from ships, aircraft, vehicles or persons, or to amateur and CB transmissions¹⁰².

The legal ground is less settled than that guide's wording suggests. RTDNA, the journalism association, points to a 2011 FCC guide noting that the Supreme Court's 2001 decision in Bartnicki v. Vopper "questions the ability of the government to regulate the disclosure of legally-obtained radio communications"¹⁰³. So the FCC's text states a divulgence ban, and the FCC itself has acknowledged that a Supreme Court ruling casts doubt on how far it can be enforced against people who obtained the audio lawfully. This guide does not try to resolve it.

Why can't a scanner pick up cell phone calls?

What you see: Scanner packaging or specs with a gap in the frequency range, or a note that cellular bands are "blocked."

That gap is required. Section 302(d) of the Communications Act bars the FCC from authorizing scanners that can receive cellular frequencies, that can be readily altered to do so, or that can decode digital cellular signals to analog¹⁰². Under 47 CFR 15.121, such receivers could not be manufactured or imported for US use after April 26, 1994, and 47 CFR 2.803 bars selling unauthorized scanners¹⁰².

Separately from the equipment rules, intercepting phone calls can bring criminal penalties under 18 U.S.C. 2510-2520¹⁰².

Can I have a scanner in my car?

What you see: A scanner on a dashboard, or a state law that says scanners in cars require a license or permit.

Federal law does not address this; some states do. The list below comes from a scanner retailer's state guide, a secondary commercial source, and the statutes themselves should be checked before relying on it¹⁰⁴.

Massachusetts does not appear on that list of in-vehicle restrictions¹⁰⁴.

Read on the states' own code sites (Indiana via Justia) as of October 2026, the statutes differ more than the table suggests:

• Florida, s. 843.16 (2026 Statutes): a first-degree misdemeanor to install or transport, in a motor vehicle or business, a receiver tuned to police, fire rescue or state agency frequencies. Exemptions include FCC amateur licensees, full-time news organizations, alarm monitoring contractors and some public safety workers.¹⁰⁵
• Indiana, IC 35-44.1-2-7: a general possession offense rather than a vehicle rule. Knowingly possessing a "police radio" - defined to cover radios used in vehicles or carried by a person, but not radios designed only for home use - is a Class B misdemeanor. Exemptions include amateur licensees who do not transmit on police frequencies, news gatherers, home or business use and written police permission; using a police radio or a police scanner app in a crime or to evade police is a separate offense.¹⁰⁶
• Kentucky, KRS 432.570: bars possessing or installing a radio that can send or receive police messages, including in vehicles, but exempts, among others, receive-only radios at home, licensed tow operators, reporters and amateur licensees. A violation is a misdemeanor with a $50 to $500 fine, up to 12 months in jail, or both.¹⁰⁷
• Minnesota, s. 299C.37 (2025 Statutes): bars equipping, using or possessing in a vehicle a receiver for a "police emergency frequency" without a permit from the superintendent of the Bureau of Criminal Apprehension. Peace officers and most FCC amateur licensees are exempt; a first offense is a misdemeanor.¹⁰⁸
• New York, VTL s. 397: bars equipping or knowingly using a vehicle with a receiver that picks up police frequencies without a local permit. It exempts licensed amateurs using amateur frequencies; the penalty is a fine up to $1,000, up to six months in jail, or both.¹⁰⁹
• Michigan, MCL 750.508: not a vehicle rule. It bars anyone convicted of a felony in the past five years from possessing a receiver for police, fire, EMS, corrections or homeland security frequencies, and bars possessing one while committing a crime.¹¹⁰

Why did my scanner go quiet, and do I need a digital one?

What you see: A scanner that used to carry local police now hissing or silent, or a listing that says "P25 Phase 2" or "encrypted."

There are three common reasons, and they need different answers:

Encryption is a feature of the P25 standard, which "supports secure communication through the use of encryption, key management, and equipment authentication"⁸⁴. Encrypted traffic is not "readily accessible to the general public" under the Wiretap Act, whose definitions exclude radio that is "scrambled or encrypted"¹¹¹, and RTDNA notes that no law requires agencies to keep their feeds publicly available¹⁰³. This guide does not cover ways around encryption.

Some agencies that encrypt offer a delayed online feed instead; Boston's is described below. For P25 basics, see the public safety section; for how dispatch works end to end, see https://works.cold.fyi/911/.

Why are police departments encrypting their radios?

What you see: A local news story about the police scanner "going dark," or a department notice about a new radio system.

By December 2023, according to New York Times reporting republished by Boston.com, Denver, San Francisco, San Diego County, Baltimore, Chicago, New York and Sioux Falls had encrypted some or all of their signals; Minneapolis planned to in 2024 and Indianapolis was considering it¹¹².

The case for encryption, as police and dispatchers put it. Minneapolis Police Chief Brian O'Hara cited a suspect search relayed live on social media, a viral false abduction report, and victims' personal information going out over the air¹¹². The same reporting describes strong support for encryption in the dispatch community, quoting Mike Hubbs¹¹². In Colorado, Greenwood Village city manager John Jackson argued that criminals monitor police traffic and that victim and witness details should not be released instantly¹¹³.

The case against, as journalists and civic groups put it. Greg Nojeim of the Center for Democracy and Technology said immediate emergency information has value to society and that privacy can be protected without cutting off access¹¹². The Rev. Charles Harrison in Indianapolis said encryption would increase distrust of police¹¹². Denver-area TV and newspaper reporters cited shootings where police were slow to warn the public, or did not warn it at all¹¹³.

Compromises in use. Las Vegas, which encrypted in 2018, gave some journalists access; Chicago kept a Broadcastify feed on a 30-minute delay; San Diego County publishes a call log without precise addresses or names¹¹².

What did Colorado decide?

What you see: References to a Colorado "media access policy" for police radio.

Colorado had one of the longest-running fights. Bills to limit encryption failed in 2018, 2019 and 2020; the last, HB 20-1282, died 6-5 in the House Transportation and Local Government Committee on March 4, 2020¹¹³. Aurora police had encrypted around 2016 and Denver police in 2019¹¹⁴.

In 2021 the rule arrived by another route, added to HB 21-1250, a police accountability bill¹¹⁴. The current statute, C.R.S. 24-6-502, says an agency that encrypts all of its radio traffic must adopt a communications access policy, developed with Colorado media, giving the media access to primary dispatch channels or talkgroups through commercially available scanners or radios or other feasible technology¹¹⁵. The policy may cover credential checks, limits on use and equipment costs¹¹⁵.

Putting it into practice was slow. As of August 2022, COLab reported that Denver-area news outlets still had no access agreements, and Denver cited an auditor's clause in its contract¹¹⁴.

What happened in California?

What you see: Bay Area departments announcing encryption in 2021, while the Highway Patrol stayed audible.

California's push came from data protection rather than officer safety. A state Department of Justice bulletin, 20-09-CJIS, dated October 12, 2020, told agencies to protect criminal justice information and personally identifiable information drawn from CLETS, the state's law enforcement data system; the options were to stop sharing that information on open channels, or to encrypt all communications¹¹⁶.

Agencies chose differently. The CHP did not encrypt, citing older radios, and since October 2020 limits personal information per transmission and sends details to in-car computers¹¹⁷. Palo Alto encrypted on January 5, 2021, and Mountain View and Los Altos followed in early 2021; in July 2021 the DOJ told Palo Alto it could not go back to broadcasting personal information unencrypted¹¹⁷. Fremont kept its primary channel open and moved sensitive channels to AES 256-bit encryption on October 15, 2025, through the East Bay Regional Communications System Authority¹¹⁶.

In Boston: why can't I hear the police on my scanner?

What you see: Boston Police dispatch missing from a scanner or a live scanner app since August 2025.

Boston Police switched from analog radio to a "digital encrypted radio system" on August 9, 2025, and analog scanners no longer receive it⁹⁰. The department gave its reasons as replacing old equipment, improving coverage, reducing static, and preventing monitoring and interference by "bad actors"⁹⁰.

BPD offers a free public feed at radio.rapidsos.com/boston, which the department says is "delayed slightly"⁹⁰; a police spokesperson told the Boston Globe the delay is about 5 minutes in most cases¹¹⁸. Justin Silverman of the New England First Amendment Coalition told the Boston Globe that most police radio traffic is routine and only sensitive traffic needs encryption¹¹⁸.

Boston's switch covers the Boston Police Department. Many neighboring towns share the regional BAPERN network described in the public safety section, and BPD's notice does not say whether their traffic, or Boston Fire and EMS, is affected; a current scanner listing for those agencies is the place to check.⁹⁰

Elsewhere in the world

What you see: A scanner sold in the UK with a leaflet warning about what you may listen to.

United Kingdom. Section 48(1) of the Wireless Telegraphy Act 2006 makes it an offense, "without lawful authority" (wording substituted by the Investigatory Powers Act 2016, in force from June 27, 2018), to use wireless telegraphy apparatus "with intent to obtain information as to the contents, sender or addressee of a message" of which you are not an intended recipient, or to disclose such information¹¹⁹. The penalty on summary conviction is a fine up to level 5¹¹⁹¹²⁰.

The test turns on whether you are an intended recipient, so the US idea that anything unencrypted is fair to hear does not carry over. Broadcast radio, aimed at everyone, is a different case from a dispatcher talking to a patrol car. UK police radio runs on Airwave, the national TETRA network described in the public safety section. The government's July 2026 ESN business case discusses the security risks of Airwave's "air interface encryption algorithm," which confirms the radio link is encrypted.¹⁰⁰ Wikipedia describes the move from open analog to the encrypted digital TETRA system as making it "practically impossible" for civilians to listen to UK police radio.¹²¹

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08How flight trackers know where every plane is

The short version: Most airliners and many small planes broadcast their own GPS position, altitude, speed and identity about once a second, unencrypted, on 1090 MHz (and, for some small US aircraft, 978 MHz). Flight-tracking sites collect those broadcasts from tens of thousands of ground receivers, most of them run by volunteers, and fill gaps by timing older transponder replies across several receivers. Owners can ask the FAA to hide a plane from public sites, but that does not stop the radio signal. In the US, federal law allows anyone to listen to aeronautical radio; the rules differ by country, and the UK's are described as stricter.

What is ADS-B, and why can anyone receive it?

What you see: A flight-tracker app showing a plane over your house with its flight number, altitude and speed, updating every second or so.

ADS-B (Automatic Dependent Surveillance-Broadcast) is the aircraft announcing itself. "Dependent" means it depends on the aircraft's own navigation, and "broadcast" means it is sent to everyone rather than in answer to a question. Under the FAA's description, ADS-B Out carries the aircraft's position, both barometric and GPS altitude, horizontal and vertical velocity, its ICAO aircraft address (tied to the tail number for US aircraft), its Mode A squawk code and a Flight ID such as an N-number or airline call sign.¹²² It updates about once per second, and the FAA's specification calls for no encryption.¹²² The position comes from GPS or WAAS (see GPS), and the equipment must transmit at all times unless the FAA or ATC directs otherwise.¹²³

That openness is why flight tracking exists as a hobby and a business: a cheap receiver hears the same broadcast the FAA hears.

ADS-B In is the receiving side in the cockpit, and it is not required.¹²² Aircraft that have it can get TIS-B traffic information on either link and, on 978 MHz only, free FIS-B weather and flight information.¹²²

What is the difference between 1090 and 978 ADS-B?

What you see: A tracking site listing a plane's data source as "ADS-B" or "UAT," or a pilot shop selling two kinds of box.

The US runs two ADS-B links. 1090ES (1090 MHz Extended Squitter) is the worldwide standard, sent by a Mode S transponder. UAT (Universal Access Transceiver) on 978 MHz is a US system.¹²⁴ Flightradar24 describes UAT as "used primarily for smaller aircraft flying only below 18,000 feet in the United States."¹²⁵

Under 14 CFR 91.225, Class A airspace (above 18,000 ft, about 5,500 m) requires 1090ES, while in the other mandated airspace an aircraft may use either link.¹²⁶ A receiver that listens only on 1090 misses the small planes using 978.

Where is ADS-B Out required in the US?

What you see: Nothing in the sky - but this table is why nearly every plane near a big airport shows up on a tracker.

The rule took effect on January 1, 2020: "After January 1, 2020, unless otherwise authorized by ATC, no person may operate an aircraft" in the covered airspace without compliant equipment.¹²⁶

Source: eCFR, 14 CFR 91.225.¹²⁶ Aircraft certificated without an engine-driven electrical system, such as balloons and gliders, are excepted in some of this airspace.¹²⁶

As of October 1, 2026, the FAA counted 173,663 US civil aircraft equipped with ADS-B Out, 164,573 of them listed as good installs, and 7,200 equipped US air carrier aircraft.¹²⁷

What does a transponder do?

What you see: A plane on a tracker with an altitude but a vague or estimated position, or labeled "MLAT."

Before ADS-B there was secondary surveillance radar, and it is still in use. The FCC's aviation frequency table lists 1030 MHz and 1090 MHz, the latter described among other uses as "Vehicle Squitter."¹²⁴ In plain terms, a ground radar asks on 1030 MHz and the aircraft's transponder answers on 1090 MHz. A Mode S transponder carries a unique address for each aircraft, so the radar can talk to one plane at a time. That address is the ICAO address, and for US aircraft the FAA ties it to the tail number.¹²² The four-digit squawk code assigned by ATC rides along too.¹²²

ADS-B 1090ES reuses that same transponder to send unprompted "extended squitter" messages. Some older civil aircraft and many military aircraft carry only Mode S transponders, which send less data, so trackers have to work out where they are by other means.¹²⁵

In the FAA's air traffic control order (JO 7110.65), Mode 3/A is "designated as the common military/civil mode for air traffic control use," and a Mode C transponder replies with the aircraft's pressure altitude in 100-foot increments.¹²⁸ The same order covers the three emergency codes in the US: 7500 for hijack or unlawful interference, 7600 for loss of two-way radio communications, and 7700 for an emergency.¹²⁸

How does Flightradar24 know where planes are?

What you see: A map full of tiny yellow planes, each one tappable, including over the open ocean.

Flightradar24 (FR24) says its primary source is ADS-B, with multilateration for aircraft that have only Mode S, satellite ADS-B over the oceans (more in satellites), and further inputs from North American and Australian radar data, FLARM and the Open Glider Network, and UAT.¹²⁵ It says it has "more than 50,000 receivers on seven continents and in space" and a database of more than 1.5 million aircraft.¹²⁵ In a blog post from April 2025, updated October 2025, FR24 said it had "recently surpassed 50,000 connected ground stations globally," of which about 7,000 are FR24-supplied kits and the rest are user-built.¹²⁹

FlightAware works the same basic way. In October 2020 it reported 30,000 ADS-B receivers in 193 countries on all seven continents and called it "the largest terrestrial ADS-B network in the world."¹³⁰ Volunteers host the receivers, and the company may give out a free FlightFeeder where it needs coverage.¹³⁰ The "About FlightAware" note on the same page gives no exact count, saying only "tens of thousands" of receivers.¹³⁰

The ground receivers only listen; nothing is sent to the aircraft.

What is MLAT, and why can't I see a plane on Flightradar24?

What you see: A plane you can hear overhead that is missing from the app, or one that appears only once it climbs.

MLAT (multilateration) finds a transmitter by comparing when the same signal reached several receivers. FR24 says it uses Time Difference of Arrival and that "Four FR24-receivers or more, receiving signals from the same aircraft, are needed to make MLAT work."¹³¹ It works only from roughly 3,000-10,000 ft (about 900-3,000 m) and above, because lower down too few receivers can hear the plane.¹³¹

So a plane can be missing for several ordinary reasons:

• It has no ADS-B and is too low, or in too thin a receiver area, for MLAT.¹³¹
• It uses 978 MHz UAT and no nearby receiver listens there.
• Its owner has asked for it to be blocked from public display (see LADD below).¹³²
• It is over the ocean, beyond the reach of ground receivers, where FR24 relies on satellite ADS-B, and it has no ADS-B to hear.¹²⁵

Coverage follows the volunteers, so it thins out wherever few of them live.

What is ADS-B Exchange?

What you see: A tracking site showing military jets and business aircraft that the bigger apps leave off.

ADS-B Exchange was "founded in 2016 ... as a hobby project."¹³³ It describes itself as offering "a transparent view of the skies without censorship or filtering," including military traffic, and like the others it uses MLAT and community-hosted receivers.¹³³ The site's footer links to JETNET's terms.¹³³

The difference is policy, not radio. FR24 and FlightAware honor the FAA's LADD blocking list; ADS-B Exchange describes its view as unfiltered.¹³³

Can owners hide a plane from flight trackers?

What you see: A private jet that shows up as a blank or generic entry, or not at all, on a mainstream tracker.

Two FAA programs exist, and neither switches off the radio.

LADD makes FAA-fed vendors hide a registration from public display. It grew out of BARR, a blocking system that NBAA, the FAA and data vendors began developing in 1997, and was later renamed LADD; NBAA welcomed new FAA privacy measures in a November 4, 2019 press release.¹³⁴ Vendors must block LADD-listed registrations, call signs and flight numbers and show no history for them.¹³⁴

LADD now operates under Section 803 of the 2024 FAA Reauthorization Act, filters data from the FAA's SWIM feed and participating websites, and offers "FAA Source" (FAA use only), "Subscriber Level" and "Unblocking" options, with changes taking effect on the first Thursday of each month.¹³² The FAA is direct about the limit: "The LADD program does not impact the ADS-B broadcast data," which "may be received by off-the-shelf receivers."¹³⁵

PIA (Privacy ICAO Address) assigns a temporary ICAO address that is not linked to the owner in the Civil Aircraft Registry.¹³⁵ To qualify, an aircraft must be US-registered, use 1090 MHz ADS-B, use a third-party call sign and fly in US-managed airspace; dual 1090/UAT aircraft were not eligible.¹³⁵ It costs nothing in the first phase and can be re-requested after 20 days, per the FAA page as updated March 22, 2025.¹³⁵

FlightAware's PIA page gives a 60-day interval for the "initial phase," which differs from the FAA's current figure.¹³⁶ PIA is valid only in US and US-managed airspace, and the aircraft must revert to its registered address when leaving it.¹³⁵

Small planes have one more option. A UAT aircraft below 18,000 ft squawking 1200 (flying under visual rules) can select an "anonymous mode" that broadcasts a randomized address, at the cost of ATC services.¹²² The FAA keeps its own ADS-B surveillance data for at least six months regardless.¹²²

The two sides, as sourced: the business-aviation group NBAA publicly commended the FAA's 2019 privacy measures,¹³⁴ while ADS-B Exchange presents an unfiltered view as transparency.¹³³

What is ACARS?

What you see: Short bursts of digital noise on a scanner around 129-131 MHz, or a website listing text messages from airliners.

ACARS is text messaging between aircraft and their airlines, and increasingly air traffic control. ARINC developed it, and it has carried routine pilot-to-airline messages since 1978.¹³⁷ VHF ACARS runs mainly on 129.125 and 131.550 MHz with roughly 200 nm (370 km) line-of-sight range; HF datalink covers 2-22 MHz, and satellite links use Inmarsat around 1.5 GHz and Iridium around 1.6 GHz.¹³⁷ The FCC table lists 131.450, 131.550, 131.725, 131.825 and 136.850 MHz for datalink.¹²⁴ Oceanic controller messaging (CPDLC) and position reporting (ADS-C) ride over ACARS in remote airspace.¹³⁷

The faster successor is VDL Mode 2, which uses twelve 25 kHz channels from 136.700 to 136.975 MHz, with 136.975 MHz as the common signaling channel. It runs at 31.5 kbit/s raw, against 2.4 kb/s for plain VHF ACARS.¹³⁸ Europe's Single European Sky rule (January 2009) required new aircraft flying in Europe after January 1, 2014 to carry CPDLC (Controller-Pilot Data Link Communications), with VDL Mode 2 as the primary link.¹³⁸

US law does not name ACARS; the relevant text is the federal wiretap law's exemption for "any marine or aeronautical communications system," covered below.¹³⁹ Elsewhere, check the national regulator's rules.

What frequency is air traffic control?

What you see: A scanner or a streaming site playing clipped, fast voices: "Delta 1234, contact departure."

Pilots and controllers talk in plain voice on the VHF airband. The US frequency table assigns air traffic control and related services in 25 kHz steps across these ranges:¹²⁴

Sources: 47 CFR 87.173, Airframes.io and Wikipedia's VHF Data Link article.^124137138

Airband voice uses AM rather than FM. AM does not suffer from FM's "capture effect," so when two stations transmit at once, listeners hear a garbled mix of both instead of only the stronger signal.¹⁴⁰ Europe has also split the standard 25 kHz channels into three, giving 8.33 kHz spacing.¹⁴⁰ EUROCONTROL says Europe has "a known shortage of voice communication frequencies," and under EU Implementing Regulation 1079/2012 aircraft without 8.33 kHz-capable radios may not be able to operate in EU member states' controlled airspace from January 1, 2018.¹⁴¹

Is it legal to listen to air traffic control?

What you see: LiveATC-style streams of tower audio, or a scanner tuned to the local approach frequency.

In the US, yes. The federal wiretap statute says it is not unlawful to intercept radio communications from stations "for the use of the general public," those relating to "ships, aircraft, vehicles, or persons in distress," and those "transmitted by any marine or aeronautical communications system."¹³⁹ That is the legal footing for scanner listeners and for sites that stream tower audio. State scanner rules and police radio are separate questions, covered in scanners.

In the UK the picture is stricter, though the sources are less direct. A 2012 Parliament petition, citing the Wireless Telegraphy Apparatus (Receivers) (Exemption) Regulations 1989, stated that "It is an offence to listen to any other radio services unless you are authorised by a designated person to do so."¹⁴² That is a petitioner's statement, not an official ruling; the petition drew 96 signatures and no government response.¹⁴² Ofcom's 2014 license-exemption notice states the general offense "to establish, install or use equipment for wireless telegraphy without holding a licence" but says nothing specific about receive-only scanning.¹⁴³

Section 48 of the UK's Wireless Telegraphy Act 2006 makes it an offense, "without lawful authority," to use wireless telegraphy apparatus "with intent to obtain information as to the contents, sender or addressee" of a message of which neither the listener nor anyone they act for is an intended recipient, or to disclose such information.¹²⁰ The penalty on summary conviction is a fine up to level 5 on the standard scale.¹²⁰ The section does not mention scanners by name.

Elsewhere in the world

What you see: The same flight-tracker apps working in Europe, but with no 978 MHz traffic.

Outside the US, ADS-B means 1090ES; UAT is a US system.¹²⁵ Europe's rule is EU Regulation 1207/2011, as amended, and it was carried into UK law after Brexit; legislation.gov.uk shows the retained text, with the airspace wording in paragraph 5 substituted from December 31, 2020.¹⁴⁴

Sources disagree on the date. OPSGROUP, writing in November 2019, listed "June 7, 2020 - European ADS-B Out Mandate" for IFR aircraft above 5,700 kg or 250 kt, requiring a GPS source coupled to a 1090ES transponder.¹⁴⁵ The current text of Article 5 says that by December 7, 2020 operators must have serviceable transponders, with Parts A and B applying to aircraft over 5,700 kg (about 12,600 lb) maximum takeoff mass or 250 kt (about 460 km/h) cruise speed first certificated on or after June 7, 1995, and fixed-wing aircraft also meeting Part C.¹⁴⁴

Operators with a retrofit program in place before that date had until June 7, 2023, and exemptions cover aircraft flying for maintenance or export or retiring by October 31, 2025, and up to 3 consecutive days with an inoperative transponder.¹⁴⁴

The main difference from the US: Europe's rule keys on aircraft weight and speed, while the US rule keys on airspace.

In Boston: why do planes fly low over certain neighborhoods?

What you see: A steady line of jets climbing out over Cambridge or Somerville, or arriving low over the harbor, on some days and not others.

Logan International, run by Massport, sits on Boston Harbor 2.5 miles (4 km) northeast of Back Bay on 2,384 acres (965 ha).¹⁴⁶ It has six runways:¹⁴⁶

Planes are low near an airport because they are lined up on final approach or have taken off along a fixed procedure, and which runway is in use mostly follows the wind. Runway 14/32 shows the wind's role plainly: it opened November 23, 2006, after being proposed in 1973, runs one way only (landings on 32, takeoffs on 14), is court-limited over land, and operates only in a strong northwest wind; residents want at least 11.5 kt and Massport favors 10 kt.¹⁴⁶ Chelsea, East Boston, Winthrop and Revere residents opposed it.¹⁴⁶

The bigger change came from RNAV, area navigation along precise GPS-defined paths. The FAA's Environmental Assessment for the runway 33L RNAV departure came out in May 2013.¹⁴⁷ Cambridge Day reported that RNAV began in June 2013, while the City of Cambridge says the procedures were implemented in 2014.¹⁴⁸¹⁴⁷

RNAV was introduced "to make flights more efficient and reduce fuel burn," and it produced "a higher concentration of flights in a smaller cluster of neighborhoods," in Cambridge Day's words.¹⁴⁸ Departures from 33L became "more concentrated over parts of north and west Cambridge," according to the city.¹⁴⁷ People under that narrow track hear more flights even if the total number has not changed.

Citing Massport data, Cambridge Day reported complaints from Medford rising from 0 in 2013 to 7,861 in 2017, Somerville from 162 to 3,840, Arlington from 6 to 2,225, and Cambridge from 263 in 2013 into the thousands by 2016.¹⁴⁸

The FAA, Massport and MIT then studied ways to spread the noise more evenly.¹⁴⁷ A 2021 MIT proposal added a departure leg over central Somerville and Porter Square, and Cambridge and most communities on the Massport Community Advisory Committee, a body created by the state legislature, voted against it.¹⁴⁷ Each proposed fix moves some of the noise somewhere else, which helps explain why proposals have drawn opponents.

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09Ships, AIS and channel 16

The short version: Large ships announce themselves on two VHF channels, 161.975 and 162.025 MHz, using AIS, the maritime cousin of the ADS-B signal that puts planes on flight trackers. An international treaty, SOLAS, requires AIS on most ships of 300 gross tonnage and up and on all passenger ships, and US rules extend it to many smaller commercial vessels. Ship-tracking sites such as MarineTraffic map those broadcasts using shore receivers mostly run by volunteers. For voice, channel 16 (156.800 MHz) is the worldwide distress, safety and calling channel, and in the US federal law lets anyone listen to marine radio, though rules differ elsewhere.

What is AIS?

What you see: A ship-tracking app showing a container ship, ferry or tug in the harbor with its name, destination and a little arrow for its heading.

AIS (Automatic Identification System) is a radio system in which a vessel announces itself on VHF. The US Coast Guard's channel table lists two channels for it: AIS 1 on 161.975 MHz and AIS 2 on 162.025 MHz, both labeled "Automatic Identification System (AIS)."¹⁴⁹ Like aircraft ADS-B (see aviation), it is a broadcast meant to be heard by anyone in range, which is why other ships, shore stations and hobbyists can all pick it up.

The idea is collision avoidance and traffic awareness: a ship's bridge sees nearby vessels by name, not only as anonymous radar blips. The same openness is what lets a website plot the world's shipping.

US rules recognize two equipment classes. Class A is the full-grade unit required on the vessels in the next table, and Class B is a lighter class that the rule allows on some fishing and smaller vessels.¹⁵⁰

VHF signals travel roughly line of sight, so a shore receiver hears ships out to its horizon and no farther.

In the US Coast Guard's summary, each AIS position report carries the ship's MMSI identity number, position, course and speed over ground and heading, and on Class A units also rate of turn and navigational status; a separate message every 6 minutes adds details such as name, call sign, ship type and dimensions, and on Class A the destination and ETA.¹⁵¹ A Class A unit transmits at 12.5 W and reports every 2 to 10 seconds under way, depending on speed and turning, and every 3 minutes when anchored or moored.¹⁵¹ The lighter Class B-CS units transmit at 2 W and report every 30 seconds.¹⁵¹

Which ships have to carry AIS?

What you see: Every big ship in a harbor shows up on a tracker; many small boats do not.

Internationally, the rule is the International Convention for the Safety of Life at Sea (SOLAS), Chapter V, Regulation 19.2.4, as compiled by the US Coast Guard:¹⁵²

New ships built on or after July 1, 2002 had to carry AIS from the start, and existing ships were phased in between 2003 and 2008.¹⁵² The Coast Guard compilation also contains a conflicting alternate deadline for the phase-in, so the exact dates for existing ships should be checked against the IMO's own text.¹⁵²

The passenger-ship line is the one with no size floor: under that wording, a small ferry on an international run falls under SOLAS whatever its tonnage.

What are the US rules for AIS?

What you see: Tugs, ferries, dredges and fishing boats on a tracker alongside the big ships.

US law goes further than SOLAS. The Maritime Transportation Security Act (46 USC 70114) requires AIS for self-propelled commercial vessels of at least 65 ft (about 20 m), among others.¹⁵² The Coast Guard's carriage rule, 33 CFR 164.46, sets out who needs a Class A unit:¹⁵⁰

Vessels that had previously been exempt had to install AIS by March 1, 2016.¹⁵⁰ That expansion is why a harbor tug or a large commuter ferry now appears on a tracking map in the US, and why a kayak, a sailboat or a small recreational motorboat usually does not: the rule is about commercial vessels, and recreational boats are not on the list.

How does MarineTraffic track ships?

What you see: A world map of colored ship icons, dense along coasts and thinner out at sea.

MarineTraffic's help center, hosted by Kpler, says the site gets AIS data through "MarineTraffic's network of coastal receiving stations," and that "The vast majority of these stations are operated by contributors," who can apply for a free receiver.¹⁵³ The model is the same one the flight trackers use: volunteers put up a VHF antenna and a receiver, the receiver listens on the two AIS channels, and the data goes to a central server that draws the map. Nothing is transmitted to the ships.

Because shore receivers reach only to the radio horizon, coverage is best where contributors live and along busy coasts. Out in the open ocean, a shore network cannot hear a ship at all.

Satellites fill that gap (more in satellites). The European Space Agency notes that Earth's curvature limits shore-based AIS to about 74 km from the coast, and that tracking ships with small satellites, which pick up AIS and relay it to ground stations, "solves this problem."¹⁵⁴

What is marine VHF channel 16?

What you see: A marine radio, a scanner or a harbor stream with a Coast Guard safety announcement or a captain calling another vessel and asking it to switch to another channel.

Marine VHF voice runs on numbered channels around 156-157 MHz. Channel 16 (156.800 MHz) is the "International Distress, Safety and Calling" channel.¹⁴⁹ Vessels hail each other on 16 and then move to a working channel so 16 stays clear for emergencies.

Source: USCG NAVCEN US VHF channel information.¹⁴⁹

Channel 13 is where ships coordinate passing and crossing, and in US waters ships longer than 20 m (about 66 ft) keep a listening watch on it.¹⁴⁹ The Coast Guard announces safety broadcasts on 16 and then reads them on channel 22A.¹⁴⁹ Channel 70 carries DSC (Digital Selective Calling), a digital signal with no voice, so on a scanner it sounds like a short data burst.¹⁴⁹

Can I listen to marine radio?

What you see: A handheld scanner at a waterfront, or an online stream of harbor traffic.

In the US, yes. The federal wiretap statute says it is not unlawful to intercept radio communications "transmitted by any marine or aeronautical communications system," and also those relating to "ships, aircraft, vehicles, or persons in distress."¹³⁹ The same exemption covers airband, which is why marine and aviation listening share a legal footing in the US. Police and fire radio follow different rules, covered in scanners.

Listening is not the same as transmitting. A scanner only receives; a marine VHF radio can also transmit, and transmitting is a separate matter with its own rules, outside the scope of this section.

In the UK, receiving is more restricted. Section 48 of the Wireless Telegraphy Act 2006 makes it an offense, "without lawful authority," to use a receiver to learn the contents of a message you are not meant to receive.¹²⁰ A 2012 petition to Parliament stated that listening to radio services you are not authorized to receive is an offense, while a 2014 Ofcom notice is silent on receive-only listening; see aviation for details.¹⁴²¹⁴³

Elsewhere in the world

What you see: The same AIS map working off the coast of Rotterdam, Singapore or Sydney.

SOLAS is an international convention, and its AIS rule is written for ships on international voyages, which is why the same big ships appear on trackers in every major port.¹⁵² Rules for smaller vessels on domestic voyages are set by each country's maritime authority. Channel 16 is labeled international in the US Coast Guard's own table.¹⁴⁹

The international channel plan comes from Appendix 18 of the ITU Radio Regulations, and the Coast Guard publishes a table adapted from it.¹⁵⁵ Some channels in that plan "are not allowed within U.S. territorial waters, but are allowed on the high seas," and the Coast Guard says the number of them reflects a shortage of VHF maritime spectrum in the US.¹⁵⁵

In Boston

What you see: A line of buoys in the water around Logan Airport, and police and Coast Guard boats patrolling near them.

Logan International sits on Boston Harbor, and the water around it is guarded: a 250-ft (about 76 m) security zone marked by 29 buoys, set up in 2002 and patrolled by the State Police, Boston Police, Environmental Police, the Coast Guard and harbormasters.¹⁴⁶

On a ship-tracking map, Boston Harbor follows the US carriage rules in the table above: commercial vessels of 65 ft or more, larger towing vessels and boats certificated for more than 150 passengers carry Class A AIS, so they appear, while most recreational boats do not.¹⁵⁰ On the radio side, the channel plan is the national one, with channel 16 for calling and distress and channel 13 for bridge-to-bridge talk among ships over 20 m.¹⁴⁹

Boston has no Coast Guard Vessel Traffic Service: as of October 2026 the Navigation Center lists 12 VTS areas, among them New York, Houston-Galveston and Puget Sound, and Boston is not one of them.¹⁵⁶

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10GPS, the city's hidden clock

The short version: GPS is a US government system of satellites about 20,200 km (12,550 miles) up, each one broadcasting where it is and what time it is by its on-board atomic clocks. Your phone times those signals to work out where it is - typically within 4.9 m (16 ft) under open sky, and worse among tall buildings. The same signals quietly set the clocks of cell networks, power grids and trading systems, which is one reason a GPS jammer has no lawful use in the US. The "atomic" clock on your wall gets its time from a different radio signal entirely, a 60 kHz broadcast from Colorado called WWVB.

Who runs GPS, and how many satellites are up there?

What you see: A blue dot on a map, and maybe a "satellites in view" number in a GPS test app.

GPS belongs to the US government. In its own words, "the U.S. Space Force develops, maintains, and operates" it, and the public site GPS.gov is run by the National Coordination Office for Space-Based Positioning, Navigation, and Timing.¹⁵⁷

The US commits to keeping at least 24 operational satellites available 95% of the time. As of July 3, 2023, there were 31 operational satellites, not counting retired spares still in orbit.¹⁵⁸ They fly in medium Earth orbit at about 20,200 km (12,550 miles) and each one circles the planet twice a day.¹⁵⁸

The baseline design is 24 slots in six evenly spaced orbital planes, four slots per plane, so that a user "can view at least four satellites from virtually any point on the planet." In June 2011 a change called "Expandable 24" made it effectively a 27-slot constellation.¹⁵⁸

The current fleet spans Block IIR (launched from 1997) to GPS III, first launched in 2018 with a 15-year design life.¹⁵⁸ The first four Navstar satellites, the start of it all, went up in 1978.¹⁵⁹

As of October 2026, the Coast Guard Navigation Center's constellation status table listed 32 GPS satellites, two of them flagged as unusable.¹⁶⁰

How does my phone know where it is?

What you see: The blue dot snapping into place a few seconds after you open a map, often with a widening circle while it thinks.

Each GPS satellite is a very precise clock with a radio attached. Block IIF satellites, for example, carry "advanced atomic clocks."¹⁶¹ The satellite broadcasts its position and the time it sent the signal. Your phone notes when the signal arrived. Radio travels at the speed of light, so the delay tells the phone how far away that satellite is.

Several such distances narrow your location to one point. This is trilateration: finding a position from measured distances to known points.

The catch is that your phone's clock is cheap quartz, nowhere near as good as the satellites'. So the phone treats its own clock error as a fourth unknown, alongside latitude, longitude and altitude. Four unknowns need four measurements, which is why the constellation is laid out so that at least four satellites are visible from virtually anywhere.¹⁵⁸

What frequencies does GPS use?

What you see: Nothing on the street. In a phone spec sheet, "dual-frequency GPS (L1 + L5)."

GPS broadcasts on three carriers in the L band, around 1.2 to 1.6 GHz:¹⁶²

Sources: frequencies from the 2006 L2C fact sheet;¹⁶² signal status from GPS.gov.¹⁶³

L1 C/A is the legacy signal almost every receiver uses. L5 is "designed to meet demanding requirements for safety-of-life transportation," and L1C is designed to work alongside other countries' satellite systems.¹⁶³ A second frequency matters because the upper atmosphere slows radio signals by an amount that depends on frequency. Comparing two lets a dual-frequency receiver correct for that ionospheric distortion.¹⁵⁷ As far back as 2006 the government said L2C would enable "lower-cost, dual-frequency civil GPS receivers."¹⁶²

As of October 2026, GPS.gov still describes both L2C and L5 as pre-operational, to be "employed at the user's own risk until it is declared operational."¹⁶⁴ Civil navigation messages have been broadcast on both signals since April 2014.¹⁶⁴

How accurate is GPS on my phone?

What you see: A blue dot with a pale circle around it. The circle is the phone's own guess at its error.

GPS.gov says a GPS-enabled smartphone is "typically accurate to within a 4.9 m (16 ft.) radius under open sky."¹⁵⁷

The government's formal promise is about the signal, not your phone. It commits to a daily global average user range error of 2.0 m (6.6 ft) or less, 95% of the time. That is how far off the satellite's own distance information is, before your receiver adds any errors of its own. The measured figure on April 20, 2021 was 0.643 m or less, 95% of the time.¹⁵⁷ At the other end, high-end dual-frequency receivers with correction services can get to a few centimeters in real time.¹⁵⁷

GPS.gov lists what makes things worse:¹⁵⁷

• satellites blocked by buildings, bridges and trees
• indoor or underground use
• signals reflected off buildings or walls
• atmospheric conditions
• receiver design and quality
• less often: radio interference or jamming, major solar storms, satellite maintenance, and noncompliant devices

It also notes that mapping errors are often mistaken for GPS errors.¹⁵⁷

Why is my GPS wrong downtown?

What you see: Walking between towers, the dot drifts across the street, lands you inside a building, or swings a block away and back.

Two of GPS.gov's degrading factors team up in dense downtowns: blockage by buildings and multipath, signals reflected off buildings or walls.¹⁵⁷ A street lined with tall buildings is called an urban canyon. From the bottom of one you see a narrow strip of sky, so fewer satellites are in direct view, and the ones you do see tend to sit in a line along the street, which makes for weak geometry.

Multipath is the sneakier problem. A signal that bounces off a glass tower before reaching you has traveled farther than the straight-line distance. The phone reads that extra distance as real, and your computed position shifts. If the direct signal is blocked entirely and only the reflection gets through, the error can be large.

Phones fight back with more satellite systems (next item), Wi-Fi and cell-tower positioning, and second frequencies. None of it fully cures a canyon.

A 2026 preprint by researchers at the University of Notre Dame and Sebelas Maret University logged the accuracy estimates Android phones report about themselves (a 68 percent confidence radius, not a check against surveyed positions).¹⁶⁵ Driving through high-rise downtown San Diego, a Samsung S22 reported a median of 10.29 m, against 4.19 m on a more suburban uptown route; walking inside a Las Vegas hotel-casino, medians ran from about 29 m to 48 m across three phones.¹⁶⁵

Is my phone using GPS, or something else?

What you see: A GPS test app listing satellites with little flags or letters: US, RU, EU, CN.

"GPS" has become the everyday word for a whole family of systems. The general term is GNSS, Global Navigation Satellite System. Four are global:

When the European Commission declared Galileo's Initial Services in 2016, 18 satellites were in orbit. The services were a free Open Service, an encrypted Public Regulated Service for government users, and a Search and Rescue service that is part of the Cospas-Sarsat distress-beacon system (more in satellites). ESA's Paul Verhoef called it "the transition from a test system to one that is operational."¹⁶⁶

In a city, more systems means more satellites in view, and better odds that four have a clear line to you between buildings. That is the practical reason modern phones listen to several at once.

Why do cell towers and power grids need GPS?

What you see: A small dome or puck-shaped antenna on a cell site or utility cabinet. (The gear on poles is covered in our guide to utility poles.)

Much of GPS's value is time, not place. A GPS receiver is a cheap way to get a clock that is traceable to official time. GPS.gov commits to timing within 30 nanoseconds of UTC as kept by the US Naval Observatory, 95% of the time, for a specialized fixed receiver.¹⁵⁷

In a 2017 briefing to the federal PNT Advisory Board, E. Powers of the US Naval Observatory laid out who depends on it:¹⁶⁷

• Cell networks: "LTE, CDMA and other Cell Phone technology uses precise timing from GPS (1 microsecond)."
• Power grids: "Small phase inaccuracy will reduce efficiencies and larger errors may result in damage to equipment and power outages."
• Finance: "inaccurate time stamps could result in unfair advantage being gained and loss of revenue."

A 2012 paper by Michael Lombardi of NIST gives the numbers behind that. CDMA base stations need clocks accurate to 3 microseconds, CDMA system time is nearly always derived from GPS, and handoffs between towers start to fail as offsets approach 10 microseconds.¹⁶⁸ On the grid, sensors called phasor measurement units, or PMUs, stamp voltage and current readings with GPS-derived time so that measurements from far-apart substations can be compared. The minimum requirement is 26 microseconds, and 1 microsecond is desired. The first prototype PMU, in 1988, used a GPS clock.¹⁶⁸

The same paper is blunt about dependence: when GPS is lost, some industrial clocks go out of tolerance within seconds, and "most will be out of tolerance in less than one hour."¹⁶⁸

Trading rules set their own tolerances. In the US, FINRA Rule 4590 requires member firms' business clocks to be synchronized within one second of the NIST atomic clock, and within 50 milliseconds for computer clocks that record events in NMS securities, including listed options, and OTC equity securities.¹⁶⁹ In the EU, Commission Delegated Regulation (EU) 2017/574 under MiFID II, which applies from January 3, 2018, caps divergence from UTC at 100 microseconds, with timestamps of 1 microsecond or better, for high-frequency algorithmic trading, and at 1 second for voice trading.¹⁷⁰¹⁷¹ The UK carried the regulation into its own law after Brexit, and the UK text on legislation.gov.uk has the same figures.¹⁷¹

Are GPS jammers illegal?

What you see: Usually nothing. Occasionally a news story about a truck driver, or a sale listing for a "privacy" device that plugs into a car lighter socket.

In the US, yes, and broadly. Jamming means drowning out a radio signal with noise so it can't be received. Spoofing means broadcasting fake satellite signals so a receiver computes a false position or time.

The FCC's 2013 order in the Newark case (below) sets out the law:¹⁷²

• 47 U.S.C. 333 says "no person shall willfully or maliciously interfere with or cause interference to any radio communications" of licensed or authorized stations.
• 47 U.S.C. 302(b) and FCC rule 2.803(g) bar making, importing, selling or using noncompliant devices.
• The FCC states jammers have "no lawful use in the United States" and are "per se illegal."

This guide does not describe how jammers or spoofers work beyond those definitions.

In the UK, section 68 of the Wireless Telegraphy Act 2006 "prohibits the use of apparatus (such as a jammer) for the purpose of interfering with wireless telegraphy," according to a UK government call for evidence published April 10, 2026.¹⁷³ The same document says the law "does not currently prohibit possession of a radiofrequency jammer specifically," and it sought views on penalties for possession.¹⁷³

What happened at Newark Airport?

What you see: A pickup truck on an airport access road. That was the whole visible story.

In August 2012, a worker was driving a company pickup at Newark Liberty International Airport with a GPS jammer in it. According to the FCC, it interfered with a ground-based augmentation system (GBAS) that the Port Authority of New York and New Jersey was testing there. GBAS is a set of ground stations at an airport that refine GPS for landing approaches. The FAA complained on August 3, 2012, and the FCC investigated on August 4.¹⁷²

The FCC's Notice of Apparent Liability against Gary P. Bojczak (FCC 13-106) was adopted August 1, 2013 and released August 2, 2013. It proposed a forfeiture of $31,875.¹⁷² A device meant to blind one tracker in one truck reached equipment meant to guide airliners.

Why are planes losing GPS over the Baltic?

What you see: A flight tracker showing a plane flying a strange zigzag, or a news story about airliners losing satellite navigation near a border.

Aviation regulators on both sides of the Atlantic say interference has grown sharply since 2022, much of it near conflict zones. (For how flight trackers get their data, see aviation.)

• EASA (Europe): "Since February 2022, there has been a notable increase in GNSS jamming and spoofing," with "the growing severity and sophistication" of incidents.¹⁷⁴ Its Safety Information Bulletin 2022-02, "GNSS Outage and Alterations Leading to Navigation / Surveillance Degradation," was first issued March 17, 2022. Revision 3 followed on July 5, 2024 and Revision 4 on July 3, 2026; Revision 4 adds standard pilot-controller phrasing and builds on a joint EASA-EUROCONTROL task force.¹⁷⁵
• FAA (US): Safety Alert for Operators 24002, "Recognizing and Mitigating GPS/GNSS Disruptions," dated January 25, 2024, cites "the increasing frequency of GPS/GNSS disruptions," often near conflict zones, military operations areas and counter-drone protection areas. Listed effects include loss of satellite-based approach capability, unreliable terrain warnings, wrong map positions and ADS-B position errors; some crews have diverted.¹⁷⁶
• FAA resource guide: According to an April 2026 report by GlobalAir.com, the FAA's GPS/GNSS Interference Resource Guide (first published December 2025, v1.1 in 2026) says the rate of GPS signal loss per 1,000 flights rose 65% in the first half of 2024 over the first half of 2023. Hot spots it names include the Eastern Mediterranean, the Black Sea, Russia and the Baltic, the India-Pakistan border, Iraq and Iran, the Korean Peninsula and the area around Beijing. It also describes a 2022 disruption near Denver International Airport traced to an unauthorized transmitter.¹⁷⁷ Those figures are GlobalAir's summary of the FAA guide.

How does an "atomic" clock set itself?

What you see: A wall clock or watch labeled "atomic" with a little tower icon that fills in overnight.

That clock contains no atom-counting hardware. It has a small radio receiver tuned to WWVB, a station run by the National Institute of Standards and Technology (NIST) near Fort Collins, Colorado, which broadcasts a time code continuously on a 60 kHz carrier.¹⁷⁸ NIST itself says these are better called radio-controlled clocks. They typically sync once overnight and run on ordinary quartz in between. WWVB covers most of the continental US by day, and coverage expands at night,¹⁷⁸ which is why the clocks try at night.

If yours won't sync, give it a night or two: that overnight window is when it listens.

What is that ticking voice on shortwave?

What you see: On a shortwave radio, a steady tick each second and a man's voice announcing "Coordinated Universal Time."

That is WWV, broadcast from the same Fort Collins area, or its sister WWVH on Kauai, Hawaii. NIST lists WWV on 2.5, 5, 10, 15 and 20 MHz and WWVH on 2.5, 5, 10 and 15 MHz.¹⁷⁸ The BIPM, the international time bureau, also lists WWV on 25 MHz.¹⁷⁹ Any shortwave receiver will pick them up (see hobby radio for listening), and you can listen without a radio by phone: 303-499-7111 for WWV and 808-335-4363 for WWVH.¹⁷⁸

Both stations were still broadcasting in 2026. NIST's outage log, last updated April 15, 2026, records one WWV outage, on August 15, 2023, and none after it, and eleven WWVH outages between September 2025 and April 2026, most of them on 2.5 MHz.¹⁸⁰ NIST's fiscal 2019 budget request had proposed ending the broadcasts from Fort Collins and Hawaii, including WWVB; it was a proposal, and Congress had the final say.¹⁸¹

In Boston

There is no widely cited Boston study of GPS error, but the Financial District and the streets around Downtown Crossing fit GPS.gov's description of trouble closely: tall buildings blocking sky and reflective walls bouncing signals.¹⁵⁷ A step into an open plaza can steady a wandering rideshare pin. More local radio is in Boston.

The best-known US jamming case, at Newark,¹⁷² happened roughly 200 miles (320 km) away.

For wall clocks, NIST says WWVB reaches most of the continental US by day,¹⁷⁸ which includes Boston, at the far end of a long path from Colorado.

Elsewhere in the world

GPS is free to use worldwide, and receivers need no per-country license. Every region also has access to GLONASS, Galileo and BeiDou (see the table above).

Radio-controlled clocks are a national affair. The stations use different frequencies, so a clock built for 60 kHz will not hear Germany's 77.5 kHz signal. The BIPM's table of time-signal stations (information received February-March 2026) lists:¹⁷⁹

Canada's CHU broadcasts on shortwave only, with no long-wave time code of its own in the table.¹⁷⁹

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11What's overhead: weather satellites, the ISS and Starlink

The short version: For decades anyone with a simple antenna could pull weather pictures straight from NOAA's polar-orbiting satellites on 137 MHz, but that era ended in 2025 when NOAA shut down its last legacy satellites. NOAA's geostationary GOES satellites still watch the Americas from a fixed spot, with GOES-19 taking over as GOES East in April 2025. The International Space Station carries a ham radio station that sometimes sends slow-scan TV pictures on 145.800 MHz, receivable on a handheld radio. And overhead at a few hundred kilometers, SpaceX's Starlink is the largest constellation in orbit, now authorized by the FCC for 15,000 second-generation satellites, some of which can talk directly to ordinary phones.

Can I still pick up NOAA weather satellite pictures?

What you see: Old tutorials showing a homemade antenna, a cheap USB radio dongle and a grainy black-and-white strip of cloud over the East Coast.

Not from NOAA's legacy satellites. NOAA's older polar-orbiting satellites, known as POES, sent pictures down through what Amateur Radio Newsline called "their 137 MHz APT transmissions."¹⁸² APT, Automatic Picture Transmission, is an analog image signal, and for years it was the classic first project for hobbyists with a software-defined radio (see hobby radio).

The shutdown came in 2025:¹⁸³

NOAA's earlier plan had NOAA-15 going first, on August 12, and NOAA-19 on August 19; the final order was reversed.¹⁸³ Ahead of the shutdown, RTL-SDR.com reported that the satellites would stay in orbit in a safe state "with the transmitters shut down."¹⁸⁴ NOAA's notice to users was direct: "Direct users should make plans to discontinue use of POES data."¹⁸²

So the satellites are still up there, and you can still see them pass on a tracking app. They have gone quiet.

Russia's Meteor-M satellites still broadcast near the same frequencies. As of 2026, the World Meteorological Organization's OSCAR database lists Meteor-M N2-3, launched June 27, 2023, with LRPT (Low Rate Picture Transmission) downlinks at 137.1 and 137.9 MHz giving "real-time availability of selected images."¹⁸⁵ LRPT is digital rather than analog, and RTL-SDR.com has published a tutorial on receiving Meteor-M LRPT images with a low-cost USB radio dongle.¹⁸⁶

What is GOES, and why does it never move?

What you see: The satellite loop on a TV weather forecast, with clouds sweeping across the whole of North America.

Those loops usually come from GOES, NOAA's Geostationary Operational Environmental Satellites. A geostationary satellite orbits once a day, matching Earth's spin, so from the ground it seems to hang in one place. That needs a very high orbit.

GOES-19 became operational as GOES East on April 7, 2025. It sits over 75.2 degrees west, about 22,236 miles (35,786 km) up, and replaced GOES-16, which became a backup. GOES-19 carries a lightning mapper and a coronagraph called CCOR-1, which watches the Sun.¹⁸⁷

Because GOES is so far away, picking up its data directly takes a dish and more equipment than the old APT setups needed.

NOAA's HRIT/EMWIN service is the usual starting point. It is a direct-to-user broadcast from the GOES-R series at 1694.1 MHz in the L-band, at a net 400 kbit/s, carrying National Weather Service warnings, charts and satellite imagery. NOAA says it can be received with a VSAT dish under 2 meters (at least 1.0 m at 10 degrees elevation or higher), a low-noise block-down converter, a satellite receiver and decoding software, with no internet connection needed.¹⁸⁸

Can I hear the International Space Station?

What you see: A bright, steady point of light crossing the sky in a few minutes, and a ham radio friend who says they've talked to it.

The ISS carries an amateur radio station run by ARISS, Amateur Radio on the International Space Station. ARISS "lets students experience the excitement of Amateur Radio by talking directly with crew members," through scheduled school contacts. It is backed by national amateur radio societies and AMSAT (amateur satellite) organizations in Canada, Europe, Japan, Russia and the US.¹⁸⁹

This guide covers listening only, and the easiest way in is an SSTV event.

ARISS lists the station's call signs as NA1SS (US), RS0ISS (Russia) and DP0ISS, OR4ISS and IR0ISS (Europe), adding that others may come into use as the station and crew change; the voice downlink is 145.80 MHz worldwide.¹⁹⁰ Talking back is another matter: in the US, an amateur station may transmit only under the control of a person named in an amateur license grant or authorized for reciprocal operation, under 47 CFR 97.5.¹⁹¹ Other countries license amateurs through their own regulators, such as Ofcom in the UK, ISED in Canada, ACMA in Australia and MIC in Japan.

What is SSTV from the ISS, and how do people receive it?

What you see: Social media posts of odd, slightly smeared pictures with "ARISS" in the corner, made by people in their backyards.

SSTV, slow-scan television, sends a still picture as a sequence of audio tones over an ordinary voice radio channel. ARISS runs occasional SSTV events, when the station transmits a series of images that anyone can capture.

Two documented events give the pattern:

• June 21-26, 2021: 145.800 MHz FM, in a mode called PD120. ARRL said the signal "should be receivable on a handheld radio with a quarter-wave whip."¹⁹²
• December 16-19, 2023: marking "the 40th anniversary of amateur radio in human spaceflight." 145.800 MHz plus or minus 3 kHz to allow for Doppler shift (the frequency appears to slide as the station races toward you and then away), again in PD-120 mode, with times "subject to change due to crew availability."¹⁹³

In practice, people tune a scanner or handheld to 145.800 MHz FM during a pass, record the audio, and run it through free SSTV decoding software, which turns the tones back into a picture. A pass lasts only minutes, so a tracking app to know when the ISS will be above the horizon helps.

Recent events have mostly kept that pattern. ARISS press releases list SSTV on 145.800 MHz in PD120 from October 3 to about October 9, 2025, marking the anniversary of Sputnik-1's launch, and from November 12 to 19, 2025, recognizing Scouting and the station's 25th year. A May 8-12, 2026 event broke it, using 437.55 MHz (plus or minus Doppler shift) in a mode called Robot36,¹⁹⁴ so check each announcement for the frequency.

How many Starlink satellites are there?

What you see: A trail of satellites moving in a line after sunset, or a flat white Starlink dish on a roof or RV.

More than 10,000 in total by early 2026 (details below), and climbing. The FCC first authorized 7,500 second-generation (Gen2) Starlink satellites in December 2022, less than SpaceX's request for nearly 30,000, at altitudes of 525, 530 and 535 km (about 330 miles).¹⁹⁵

On January 9, 2026, the FCC approved "an additional 7,500 Gen2 Starlink satellites," for a total of 15,000.¹⁹⁶ According to Broadband Breakfast, more than 5,000 Gen2 satellites were in orbit as of early January 2026, citing astronomer Jonathan McDowell, and the new order allows altitudes as low as 340 km (about 210 miles).¹⁹⁷

Those counts cover Gen2 only. For the whole constellation, EarthSky reported on March 22, 2026 that "there are now more than 10,000 Starlinks orbiting Earth," after a March 17 launch of 25 more, and pointed readers to Jonathan McDowell's regularly updated totals.¹⁹⁸ Tracker sites disagree with each other, so treat any single headline number with care.

What frequencies does Starlink use?

What you see: Nothing, unless you own a dish.

The 2022 authorization covered "frequencies in the Ku- and Ka-band,"¹⁹⁵ both microwave bands. The January 2026 order widened that to operations across the Ku-, Ka-, V-, E- and W-bands.¹⁹⁶

The new bands granted in 2026, per Broadband Breakfast:¹⁹⁷

These are very short waves, millimeters to a couple of centimeters long. They carry a lot of data but need a clear view of the sky, which is why a dish under a tree or beside a tall building struggles.

How can my phone text from space?

What you see: A phone with no bars showing "SAT" or a satellite icon, and a text that still goes through from a trailhead.

That is direct-to-cell: satellites that talk to ordinary mobile phones using mobile-network spectrum, with no dish. The FCC's January 2026 order covers "direct-to-cell connectivity outside the United States and supplemental coverage within the U.S."¹⁹⁶

In the US, T-Mobile offers T-Satellite, built with SpaceX. According to a June 2025 report, it was set to launch July 23, 2025 with texting (SMS), plus picture messages and audio clips on Android, with data for select apps from October 1, 2025. At the time it used 657 Starlink direct-to-cell satellites, covered more than 500,000 square miles with little or no cell coverage, and was offered to AT&T and Verizon customers with compatible phones for $10 a month.¹⁹⁹

The service is aimed at places towers don't reach.

As of July 2026, SatelliteInternet.com reported that T-Satellite launched commercially in July 2025 and costs $10 a month as an add-on, or comes included on T-Mobile's Experience Beyond plan; AT&T and Verizon customers can add it with an unlocked, compatible phone and an open eSIM slot. Features then included texting, location sharing and data for apps such as WhatsApp, Google Maps, AllTrails and AccuWeather, and T-Mobile customers on any plan could text 911 for free.²⁰⁰

In Boston

GOES East sits over 75.2 degrees west,¹⁸⁷ only a few degrees west of Boston's longitude, so from Boston it hangs in the southern sky, nearly due south.

The ISS and Starlink pass over Boston as they do over most populated places, and the ISS's 145.800 MHz downlink¹⁹³ can be heard from a city park as well as a backyard, given a clear stretch of sky toward the pass. Tall buildings downtown block low passes in the same way they block GPS satellites.

For the cell networks that direct-to-cell is designed to supplement, see cellular.

Elsewhere in the world

ARISS is an international effort, backed by amateur radio and AMSAT groups in Canada, Europe, Japan, Russia and the US,¹⁸⁹ and the 2023 SSTV event guide quoted here came from AMSAT-UK.¹⁹³ SSTV events are receivable wherever the ISS passes.

Starlink's direct-to-cell service is authorized by the FCC for use outside the US as well as for supplemental coverage inside it.¹⁹⁶ Abroad, it depends on a local partner carrier and national regulator, so check what your carrier offers.

GOES East's position over 75.2 degrees west¹⁸⁷ keeps it over the Americas. Other regions are watched by their own geostationary satellites: a Japan Meteorological Agency table (as of December 2022) lists EUMETSAT's Meteosat satellites at 0 and 9.5 degrees east, Japan's Himawari at 140.7 degrees east, China's Fengyun, Russia's Elektro-L and India's INSAT.²⁰¹

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12Wi-Fi, Bluetooth and the free-for-all bands

The short version: Your router, earbuds, smart plugs and a growing number of city sensors all transmit without anyone holding a license. In the US that is allowed under the FCC's Part 15 rules, on three conditions: you get no right to any frequency, you must not cause harmful interference, and you must put up with whatever interference you receive. Much of this traffic shares the same few "ISM" bands with machines that were never meant to communicate at all, starting with the microwave oven in your kitchen.

What does "must accept any interference received" mean?

What you see: A line of small print on the label or in the manual of almost any wireless gadget.

That sentence is the deal behind Part 15, the section of US federal rules that lets ordinary people run low-power radios with no license. Under 47 CFR 15.5, the operator of a Part 15 device "shall not be deemed to have any vested or recognizable right to continued use of any given frequency."³⁴ Operation is allowed only on condition that "no harmful interference is caused," and the device must accept interference from licensed stations, from other unlicensed radios and from industrial, scientific and medical equipment.³⁴ If an FCC representative tells you your device is causing interference, you "shall be required to cease operating the device," and you may not switch it back on until the problem is fixed.³⁴

In practice that means nobody owns the channel your Wi-Fi is on. Your neighbor's router, a wireless camera down the hall and a phone hotspot on the bus are all equally entitled to it, and none of them can complain to the government that the others are in the way. Licensed users, such as a TV station or a cellular carrier, are in a different position: they paid for or were assigned protection, and Part 15 devices must stay out of their way.

Every rule here is federal. Massachusetts does not license or regulate unlicensed radio separately, so the rules in Boston are the same as in Boise.

What is an ISM band?

What you see: Nothing, but your Wi-Fi, Bluetooth and smart-home gear all cluster on a handful of the same frequencies.

An ISM band is a slice of spectrum set aside internationally for industrial, scientific and medical machines that use radio energy for something other than communication, such as heating. Unlicensed radios are allowed to share these bands as guests, and the rule for guests is blunt: "communications equipment operating in ISM bands must tolerate any interference generated by ISM applications."²⁰²

The International Telecommunication Union (ITU) splits the world into three regions, and not every ISM band exists everywhere:²⁰²

That regional split is why a US smart-home lock or LoRa sensor built for 915 MHz is the wrong product in Europe, which uses 863-870 MHz for the same jobs instead.²⁰³

The 2.4 GHz band became the home of consumer wireless after a 1985 FCC ruling opened it to unlicensed spread-spectrum radios.²⁰⁴

In the US, the spread-spectrum rule is 47 CFR 15.247. It lets frequency-hopping and digitally modulated radios operate in 902-928 MHz, 2400-2483.5 MHz and 5725-5850 MHz, with a maximum peak conducted output power of 1 watt for most systems and less for frequency-hoppers that use fewer channels.³⁵ Machines such as microwave ovens fall under a separate rule, Part 18, which lists ISM frequencies including 915 MHz (plus or minus 13 MHz), 2450 MHz (plus or minus 50 MHz) and 5800 MHz (plus or minus 75 MHz),³⁶ and gives ISM equipment on those frequencies "unlimited radiated energy" within the band.²⁰⁵

Where did Wi-Fi come from?

What you see: The fan-shaped Wi-Fi icon on every phone, laptop and coffee shop window.

Wi-Fi's family tree goes back to 1971, when ALOHAnet linked the Hawaiian Islands with UHF packet radio.²⁰⁴ The modern standard is IEEE 802.11. Its first version came out in 1997 and topped out at 2 Mbit/s; 802.11b followed in 1999 at 11 Mbit/s.²⁰⁴

The catchy name came from the branding firm Interbrand and was in use by 1999.²⁰⁴ The industry group that certifies devices started as the Wireless Ethernet Compatibility Alliance (WECA), began promoting standardized Wi-Fi in 1999, and is now the Wi-Fi Alliance.²⁰⁶ When it marked 25 years in 2024, the Alliance counted more than 21 billion Wi-Fi devices in use.²⁰⁶

What do Wi-Fi 4, 5, 6 and 7 mean?

What you see: A number on a router box, or a small "6" or "7" inside the Wi-Fi icon on some phones.

For two decades, Wi-Fi versions went by IEEE letter codes that meant little to buyers. In October 2018 the Wi-Fi Alliance switched to generation numbers.²⁰⁷

The numbering for Wi-Fi 1 through 6 comes from the Alliance's 2018 announcement.²⁰⁷ Wi-Fi 7 brings three headline changes: channels 320 MHz wide (available only in the 6 GHz band), Multi-Link Operation, which lets a device use more than one band at once, and 4K QAM, a denser way of packing bits onto the signal that the Alliance says is about 20% faster than the 1024 QAM used before.²¹⁰ At launch, the analyst firm IDC forecast 233 million Wi-Fi 7 devices would ship in 2024.²¹⁰ Whether you need it depends on whether your other devices and your internet connection can make use of the extra capacity.

Why does my router have 2.4, 5 and 6 GHz bands?

What you see: Two or three network names from one router, or a setting called "band steering."

Each band is a trade-off. Lower frequencies travel farther and pass through walls better; higher frequencies have more room for wide channels and fewer neighbors crowding in.

• 2.4 GHz is the worldwide ISM band.²⁰² It is the most crowded: Wi-Fi shares it with Bluetooth, Zigbee, Thread and microwave ovens.
• 5 GHz came with 802.11a.²⁰⁴ Its top end overlaps the 5.725-5.875 GHz ISM band.²⁰²
• 6 GHz is the newest. In April 2020 the FCC made 1,200 MHz of spectrum, 5.925-7.125 GHz, available for unlicensed use.²¹²

The 6 GHz band was not empty. Utilities, public safety agencies and network backhaul operators use fixed microwave links there, so the FCC set two tiers. Low-power indoor devices may use the whole band. Higher, "standard-power" devices may use 850 MHz of it only under Automated Frequency Coordination (AFC), a database service that tells an access point which channels are safe given the licensed links nearby.²¹²

In the US, 5 GHz Wi-Fi runs under the FCC's U-NII (Unlicensed National Information Infrastructure) rules, which cover 5.15-5.35 GHz and 5.47-5.895 GHz below the 6 GHz band.³⁷ Power limits are set separately for 5.15-5.25, 5.25-5.35, 5.47-5.725, 5.725-5.850 and 5.850-5.895 GHz.²¹³

Can I use 6 GHz Wi-Fi outside the US?

What you see: A router or phone that offers 6 GHz at home but not abroad, or fewer 6 GHz channels in Europe.

Countries split on how much of the band to open. As of December 2021, the US, Canada, Brazil, Saudi Arabia and South Korea had opened the full 1,200 MHz, along with several other Latin American countries. The EU, by Decision 2021/1067 in June 2021, opened only the lower 480 MHz (5945-6425 MHz), and Australia and New Zealand were consulting on the lower 500 MHz.²¹⁴

The UK already allows Wi-Fi in 5925-6425 MHz. In July 2026, Ofcom decided to allow AFC-controlled higher-power Wi-Fi across the upper band, 6425-7125 MHz, with regulations expected in autumn 2026, and on July 20, 2026 it opened a consultation proposing low-power and very-low-power Wi-Fi in 6425-6585 MHz.²¹⁵ Ofcom's documents also cover mobile use of the band, which is the other contender for that spectrum in Europe.²¹⁵

Japan's regulator, MIC, sets its own 6 GHz rules, so check its current position before relying on 6 GHz there.

Why is Bluetooth called Bluetooth?

What you see: The angular blue "B" icon, and your earbuds reconnecting the moment you open the case.

Bluetooth is a short-range radio in the 2.4 GHz band. Its Special Interest Group (SIG) formed in February 1998 with Ericsson, Nokia, IBM, Toshiba and Intel as founders.²¹⁶ The name honors Harald Blåtand, "Bluetooth" in English, a Danish king who reigned roughly 940-981 AD.²¹⁶

Classic Bluetooth hops pseudo-randomly among 79 channels in the 2.4 GHz band, which helps it dodge Wi-Fi and microwave ovens sharing the same air.²¹⁶ Bluetooth Low Energy (BLE), introduced with version 4.0 in December 2009 and once called "Wibree," uses 40 channels and is built for devices that run for months on a coin cell.²¹⁶ Bluetooth 5.0 in 2016 quadrupled range.²¹⁶

Because 2.4 GHz is an ISM band worldwide, Bluetooth works the same in Boston, London and Tokyo.²⁰²

What is a Bluetooth beacon?

What you see: A store app that pops up a coupon near a certain aisle, or a small plastic puck stuck high on a wall.

A beacon is a BLE transmitter that repeats an identifier over and over. Apple introduced its iBeacon format at WWDC in 2013. Each beacon broadcasts a UUID plus two numbers called Major and Minor, and an app that recognizes them can estimate whether the phone is Immediate, Near or Far.²¹⁷ Apple switched on iBeacons in its 254 US stores on December 6, 2013.²¹⁷

The beacon itself only announces; it does not receive. Any location inference happens in the phone, through apps that have been given permission to listen, and that is where privacy questions about beacons arise.

What are Zigbee, Z-Wave, Thread and Matter?

What you see: A logo on a smart bulb or door lock box, or an app asking you to "add a Thread border router."

Smart-home sensors usually skip Wi-Fi in favor of low-power mesh radios, where each mains-powered device relays messages for its neighbors so data can hop around walls to reach the hub.

Z-Wave came from the Danish company Zensys. Sigma Designs bought Zensys in 2008 and sold Z-Wave to Silicon Labs in 2018; the Z-Wave Alliance dates to 2005.²¹⁹ Because its frequency differs by region, a US Z-Wave lock will not talk to a European hub.

Thread comes from the Thread Group, formed in July 2014 by members including Nest Labs, Samsung, ARM, Silicon Labs, Qualcomm and Yale.²²⁰ It uses the same 802.15.4 radio as Zigbee but carries internet-style IPv6 addresses (via a compression layer called 6LoWPAN), and a border router connects the Thread mesh to your Wi-Fi or Ethernet.²²⁰

Matter is the shared language on top. Version 1.0 and its certification program launched October 4, 2022, from the Connectivity Standards Alliance, "formerly the Zigbee Alliance," whose members include Amazon, Apple, Comcast, Google, Samsung SmartThings and Signify.²²¹ Matter does not have its own radio: it runs over Ethernet, Wi-Fi or Thread, and uses BLE only to commission new devices.²²¹ The idea is that a Matter light switch works with any brand's app or speaker.

On "Zigbee or Z-Wave?": Z-Wave's sub-GHz signal avoids the crowded 2.4 GHz band, while Zigbee and Thread share 2.4 GHz with Wi-Fi but have the newer Matter ecosystem behind them.

What is LoRa, and how far does it reach?

What you see: A small white box with a stubby antenna on a parking meter, a farm fence or a water tank; or "LoRaWAN" on a sensor spec sheet.

LoRa trades speed for distance. It is a proprietary form of chirp spread spectrum co-developed by Cycleo and Semtech, sending about 0.3-27 kbit/s with a practical range around 10 km (6 miles).²²² Semtech's own figures are up to 4.8 km (3 miles) in a city and 16 km (10 miles) in the country.²²² That is enough for a soil-moisture reading or a "trash bin full" alert, not for a photo.

LoRaWAN is the networking protocol on top, and it has a plan for each region: US902-928 (US915) in the US, EU863-870 (EU868) in Europe, plus AU915-928, four AS923 variants, IN865-867, KR920-923, CN470-510 and EU433.²⁰³ Europe also limits how much of the time a device may transmit on 868 MHz (its duty cycle). CEPT's 2001 decision on non-specific short-range devices, ERC/DEC/(01)04, set 25 mW and under 1% of each hour in 868.0-868.6 MHz, under 0.1% in 868.7-869.2 MHz, and 500 mW and under 10% in 869.4-869.65 MHz.²²³ Later European rules have revised parts of this framework, so check the current national rules.

What is the Helium network, and is it legit?

What you see: A crypto-mining "hotspot" box in a friend's window, or ads for Helium Mobile.

Helium is a LoRaWAN network built by the public: people buy and plug in hotspots and earn tokens in return. The company began in 2013 and its network launched in 2017 under Amir Haleem, using LoRaWAN for devices and the HNT token for rewards.²²⁴ It moved to the Solana blockchain in April 2023 (a change known as HIP 70).²²⁴ Nova Labs launched Helium Mobile in 2023, a phone service using T-Mobile plus Helium hotspots, and in June 2026 announced its sale to Noble Mobile.²²⁴

The project has a disputed record. In July 2022, reports said Helium had listed Lime and Salesforce as partners without formal relationships; Haleem disputed this in December 2023.²²⁴ The SEC sued Nova Labs on January 17, 2025, alleging unregistered securities and misleading partnership claims. On April 10, 2025, the SEC dropped the securities claims, and Nova Labs agreed to a $200,000 settlement without admitting or denying the allegations.²²⁴ Readers can weigh that record for themselves; the radio side is standard LoRaWAN.

By the project's own count in a December 19, 2025 year-in-review post, Helium had more than 120,000 active hotspots across the US and Mexico, carried 15.72 petabytes of data in 2025 and logged more than 600,000 Helium Mobile sign-ups.²²⁵

Why does my Wi-Fi drop when the microwave is on?

What you see: A video call that stutters, or a speaker that cuts out, every time someone reheats coffee.

The microwave oven is "the most commonly encountered ISM device," and it runs at 2.45 GHz,²⁰² right in the middle of the band your 2.4 GHz Wi-Fi uses. A home oven pumps about 1,000 W into the food.²²⁶ The metal box and door screen hold almost all of it in, and US rules cap what may leak out at a tiny fraction of that power (the FDA limit is below).²²⁷ That small leak is still loud next to a Wi-Fi signal that has crossed two rooms.

The oven's energy overlaps 2.4 GHz Wi-Fi channels 7 through 11.²²⁶ In a March 2009 test, a running oven cut throughput by 21% on channel 1 and 51% on channel 9.²²⁶ Under the ISM bargain, Wi-Fi has no grounds for complaint: it is the guest.²⁰² Practical fixes follow from the numbers: move the router away from the kitchen, pick channel 1 for 2.4 GHz, or use the 5 or 6 GHz band, which the oven does not touch.

You may have heard that 2.45 GHz is the "resonant frequency of water." It is not: physicist Christopher Baird of West Texas A&M University writes that oven microwaves "are not tuned to a resonant frequency of water," and that the frequency is used largely because regulators allocated it to ovens.²²⁸

In the US, oven leakage is an FDA rule, not an FCC one. Under 21 CFR 1030.10, which applies to ovens made after October 6, 1971, leakage may not exceed 1 milliwatt per square centimeter at any point 5 cm or more from the oven's external surface before the oven is sold, and 5 milliwatts per square centimeter at any such point over the rest of its life.²²⁷ For questions about RF exposure and health, see health.

In Boston

Unlicensed radio is the same in Boston as anywhere else in the US, because the rules are federal.³⁴ Two local touches: the 902-928 MHz band that Boston-area LoRa sensors and smart meters use exists only in the Americas,²⁰² and MassDOT has used Bluetooth radios to time traffic on state highways (see everyday radios).

Elsewhere in the world

• UK and EU: The equivalent of Part 15 is called "licence-exempt" use, with a similar no-protection principle plus power and duty-cycle limits. Europe uses 433 MHz and 863-870 MHz where the US uses 902-928 MHz.²⁰²²⁰³
• Canada, Australia and Japan: ISED, ACMA and MIC each set their own license-exempt rules, with their own bands and power limits; check the national regulator before using imported gear.
• 6 GHz: The EU opened only 5945-6425 MHz; the UK is extending into the upper band in 2026.²¹⁴²¹⁵
• Smart home: Z-Wave moves to 868.42 MHz and sub-GHz Zigbee to 868 MHz in Europe; 2.4 GHz Zigbee, Thread and Matter are the same worldwide.²¹⁹²¹⁸

For how event crews share crowded spectrum at a show, see show wireless.

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13The radios you carry without knowing

The short version: Plenty of transmitters in your life have no screen and no Wi-Fi icon. The electric meter on your house radios its readings to the utility, each tire on a modern car reports its own pressure, the card or phone you tap at a subway gate talks to the reader over a few centimeters of air, and a toll tag answers every gantry you drive under. In Massachusetts, both big electric utilities are swapping in radio-equipped meters through 2027, and since August 2024 the MBTA has taken a bank card or phone directly at the gate.

What is that new electric meter, and does it transmit?

What you see: A meter with a digital display and no spinning dial, sometimes with a sticker or door hanger saying it was upgraded.

These are AMI meters, short for advanced metering infrastructure: meters that report usage over a two-way network instead of waiting for someone to walk by and read them. A common US design is a radio mesh in the 902-928 MHz band (one of the unlicensed ISM bands). In Itron's OpenWay system, documented in a white paper published by the Canadian utility BC Hydro, each meter sends its own data one to three times a day and also relays its neighbors' data, hopping toward routers mounted on utility poles that carry it back to the utility.²²⁹ Transmit power is "one watt or less."²²⁹

The meters are quiet most of the time. Across 12,969 BC Hydro meters, the average time spent transmitting was 2.99 minutes per day, a duty cycle (the fraction of time a transmitter is actually sending) of 0.21%.²²⁹ Those figures describe BC Hydro's network; other utilities and meter generations may differ.

Some people object to having a transmitter on their home, on health or privacy grounds, and utilities offer opt-outs in some places (see below). This section takes no side; what health agencies and researchers say about RF exposure is in health.

When is Eversource installing smart meters in Massachusetts?

What you see: A letter or text from Eversource scheduling a meter swap.

Eversource is replacing its Massachusetts meters with Itron Gen 5 meters, installed by the contractor Grid One Solutions, in a project expected to take about three years.²³⁰ Installs began in July 2025 in Springfield, with about 1.5 million meters to upgrade.²³¹ By October 31, 2025, more than 35,000 meters had gone in across western Massachusetts; eastern Massachusetts was to follow in early 2026 after network upgrades, with a target of up to 60,000 meters a month by spring 2026.²³¹

On January 13, 2026, Eversource reported 100,000 meters installed out of more than 1.5 million planned, said the communications devices in eastern Massachusetts were "more than 75 percent complete," and repeated a finish date of late 2027.²³² Eversource also cites a figure that more than 76% of US meters are already smart.²³¹

Eversource has said little in public about the radio itself. A May 15, 2025 Eversource notice posted by the Town of Chilmark says the meter "doesn't rely on Wi-Fi" but on "a safe and secure Eversource network,"²³³ and Eversource's smart meter safety page, which does not say which service area it covers, lists the meter's frequency as "902 -2000 MHz."²³⁴

The same notice set the eastern Massachusetts schedule: network equipment between August 2025 and the end of July 2026, and meter replacement "between March 2026 and the end of 2027."²³³

Can I opt out of a National Grid smart meter in Massachusetts?

What you see: A National Grid notice in the mail, or a town website post about upcoming meter work.

National Grid is running the same kind of program. As of April 23, 2026, it had installed more than 500,000 smart meters in Massachusetts, over 35% of its target, and plans to reach all 1.4 million electric customers by the end of 2027.²³⁵ One feature it highlights: the new meters signal the control room when they lose power, so the utility can see an outage without waiting for phone calls.²³⁵

There is an opt-out. According to a notice posted by the Town of Salisbury ahead of a May 21, 2026 resource fair, residential customers on National Grid's R-1 and R-2 rates can ask for a non-communicating meter that is read by hand, for a fee, at ngrid.com/optout.²³⁶ The notice does not state the fee amount.²³⁶

Eversource offers an opt-out as well. In May 2025 it said residential customers who keep a manually read meter pay $34 a month, plus a one-time $42 service fee,²³⁷ and in October 2025 it said it would waive the $42 fee for customers who opt out within the 90-day notice period before installation.²³⁸

What frequency do tire pressure sensors use?

What you see: An amber dashboard light shaped like a horseshoe with an exclamation point, usually on the first cold morning of fall.

Each wheel on a modern car holds a small battery-powered transmitter that reports that tire's pressure. This is the tire pressure monitoring system (TPMS). In the US, Federal Motor Vehicle Safety Standard 138, adopted under the TREAD Act, requires TPMS on passenger vehicles, trucks and buses from the 2008 model year, meaning those built after August 31, 2007.²³⁹ The system must warn the driver when a tire is 25% or more below the pressure on the door-jamb placard.²³⁹

The sensors send short frequency-shift-keyed bursts, about 7.5 milliseconds each, at 315 MHz or 434 MHz; 434 MHz is the European choice.²⁴⁰ One widely used sensor chip from Freescale supports both frequencies.²³⁹ The European frequency sits in the 433.05-434.79 MHz ISM band, which exists only in ITU Region 1 (Europe, Africa and the Middle East).²⁰²

Air in a tire loses pressure as it cools, which is why the light tends to come on with the first frost.

Car key fobs send similar short bursts. According to an Analog Devices overview, remote keyless entry systems in the US and Japan most commonly use 315 MHz, and in Europe 433.92 MHz, with Europe also opening 868 MHz.²⁴¹ In the US, fobs, garage door openers and similar remotes fall under 47 CFR 15.231, which covers periodic transmitters at 40.66-40.70 MHz and above 70 MHz sending control signals "such as those used with alarm systems, door openers, and remote switches." Continuous transmissions, voice, video and radio control of toys are not permitted, and a manually operated transmitter must shut off within 5 seconds of being released.²⁴²

How does tap-to-pay work, and how close do I need to be?

What you see: The four curved lines of the contactless symbol on a card, phone screen or fare reader.

Transit cards, tap-to-pay bank cards and phone wallets use NFC (near-field communication) or closely related contactless-card standards, usually at 13.56 MHz.²⁴³ That frequency sits in a worldwide ISM band, 13.553-13.567 MHz.²⁰²

The card has no battery. The reader creates a magnetic field, and the card's coiled antenna picks up enough energy by induction to power its chip and answer.²⁴³ That is why range is so short. "Proximity" cards built to the ISO 14443 standard work up to about 10 cm (4 inches), while "vicinity" cards under ISO 15693, used more for tags and access badges, can reach more than 1 m (3 feet).²⁴³ The short range is a feature: you have to bring the card to the reader on purpose.

This is one kind of RFID (radio-frequency identification). RFID tags can be passive, powered by the reader like a tap card, or active, with their own battery, like a toll tag.

Is the CharlieCard being replaced?

What you see: The green CharlieCard, or a fare gate at an MBTA station with a reader that accepts phones and bank cards.

The CharlieCard went into service on December 4, 2006, replacing the token on the MBTA.²⁴⁴ It uses NXP's MIFARE Classic chip.²⁴⁴ In March 2008, Karsten Nohl's team reported that MIFARE Classic's security was broken.²⁴⁴ That August, the MBTA sued a group of MIT students (MBTA v. Anderson) over a DEF CON 16 talk; on August 19, 2008, the court allowed the talk to go ahead.²⁴⁴

The replacement is arriving in stages under the MBTA's fare program known as AFC 2.0. The MBTA board approved a $723 million contract with Cubic in November 2017, revised to more than $900 million in December 2019.²⁴⁴ Contactless bank cards and phone wallets started working at the gates on August 1, 2024.²⁴⁴ By January 2025, the Boston Globe reported, "tap to ride" had passed 10 million taps and covered every bus, every Green Line and Mattapan trolley, and every gated subway station; contactless payments rose from 7% of fares in the first month to 16% by December 2024.²⁴⁵ Public testing of a new CharlieCard began in December 2025.²⁴⁴

So the answer is yes, gradually: a new CharlieCard is in testing, and you can already skip the card and tap a bank card or phone.

As of October 7, 2026, MacRumors reported that the MBTA was testing a new CharlieCard that also works in Apple Wallet on iPhone and Apple Watch, with testing limited to riders who signed up in 2024 and a public release due "at a later date."²⁴⁶

More on how the MBTA's fare system and stations work is in How a City Works.

How does E-ZPass work?

What you see: A gray plastic box behind your rearview mirror, and steel gantries over the highway with no toll booths underneath.

E-ZPass tags are active RFID transponders: they carry their own battery, unlike a tap card. They work at 915 MHz, in the 902-928 MHz band available only in the Americas, using a time-division protocol at 500 kbit/s, and have historically been made by Kapsch (formerly Mark IV).²⁴⁷²⁰² In plain terms, the gantry reads the tag's ID as the car passes underneath, and the toll is charged to the account linked to that tag.

E-ZPass was first deployed on the New York State Thruway, at the Spring Valley toll plaza, on August 3, 1993, and by December 1996 it covered all of the Thruway's fixed-toll barriers.²⁴⁷ As of 2024 the network covered 20 states, including Massachusetts, where the older Fast Lane system was rebranded E-ZPass in 2012.²⁴⁷

How do highway signs know the travel time?

What you see: An overhead message board reading something like "BRAINTREE 14 MIN."

In Massachusetts, many of those numbers have come from radios in passing cars. MassDOT first installed its travel-time system, which relies on roadside Bluetooth readers, in summer 2012.²⁴⁸ Each reader logs the MAC address, a hardware ID that a Bluetooth radio broadcasts, of devices going by, along with the time and place. When the same address shows up at two readers, the gap between them is a travel time, and a server takes the median of many such pairs.²⁴⁹ The system wants at least 3 signals in 15 minutes and 36 matched pairs an hour to post a reliable figure.²⁴⁹ By July 2013 it covered the route from Boston to Cape Cod, using TrafficCast's BlueTOAD readers.²⁴⁹

A 2014 expansion plan called for 132 travel-time signs fed by 121 Bluetooth readers, reaching all state highways by the end of that year.²⁴⁸

A reader logs a hardware address, not a name or phone number. Whether that makes the data anonymous is a question on which transportation agencies and privacy advocates can disagree. Turning Bluetooth off on your phone and car keeps them from being counted.

In Boston

• Meters: Eversource and National Grid both serve Massachusetts and both aim to finish by the end of 2027; Eversource said its eastern Massachusetts installs would start in early 2026.²³²²³⁵
• Transit: CharlieCard since 2006, tap-to-ride with bank cards and phones since August 1, 2024, and a new CharlieCard in testing since December 2025.²⁴⁴
• Roads: E-ZPass replaced the Fast Lane name in 2012,²⁴⁷ and MassDOT's Bluetooth readers have fed highway travel-time signs since 2012.²⁴⁸

For the city's other transmitters, from TV towers to harbor radio, see Boston.

Elsewhere in the world

• Tire sensors: Europe uses 434 MHz, in an ISM band that exists only in ITU Region 1; North America mainly uses 315 MHz.²⁴⁰²⁰²
• Toll tags and meters: The 902-928 MHz band used by E-ZPass and US meter meshes is an ISM band only in the Americas, so the same hardware does not carry over to Europe.²⁰²
• Bluetooth travel times: ENR reports that New Jersey's DOT uses the same technique, fused with E-ZPass reads, as does Philadelphia.²⁴⁹
• Tap cards: 13.56 MHz is a worldwide ISM band, so contactless cards and phones use the same frequency in London, Tokyo and Boston.²⁰²

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14Wireless mics and the fight for stage spectrum

The short version: In the US, most wireless microphones and in-ear monitors live in the gaps between TV stations, as second-class tenants who must protect broadcasters. Licensed users get more power under FCC Part 74; everyone else is held to 50 mW in the TV bands. Twice, starting in 2010, the FCC has sold chunks of that TV spectrum to mobile carriers, first the 700 MHz band and then most of the 600 MHz band, and mics built for those ranges are now illegal to operate in the US. At a Super Bowl, roughly a thousand frequencies are reported in use, and coordinators plan them as a set.

Who is allowed to put a microphone on the air?

What you see: A belt-pack transmitter clipped to a presenter, a handheld mic with no cable, a performer with a molded earpiece.

A wireless mic is a small radio transmitter, and in the US it falls into one of two buckets. The licensed bucket is FCC Part 74 Subpart H, which calls these devices low power auxiliary stations (LPAS). Eligible users can get LPAS licenses for unused TV channels, but even licensees "only have secondary status" and must protect primary broadcast operations.²⁵⁰

Eligibility used to be narrow: broadcasters and film and TV producers. In 2014 (FCC 14-62) the FCC added "large venue owner or operators" and "professional sound companies" that routinely use 50 or more LPAS devices at major events.²⁵⁰ A church, a school theater or a wedding band will usually be in the other bucket.

That other bucket is unlicensed operation under Part 15 of the FCC rules (section 15.236), the same framework covered in unlicensed. Unlicensed mics run at lower power, may not cause harmful interference, and must accept interference from others.²⁵⁰ An in-ear monitor (IEM), the wireless earpiece that sends a performer their own mix, follows the same rules; it is a transmitter too, pointed the other way.

The practical difference is power and standing. A licensed user has some recourse if someone stomps on their frequency. An unlicensed user has none.

What is the 50 mW rule?

What you see: "50 mW" printed on a spec sheet or in a manual's regulatory notice.

Power here is measured as EIRP (effective isotropic radiated power): the transmitter's output including antenna gain, counted as if it radiated equally in every direction. The FCC's Office of Engineering and Technology laid out the limits in a November 2017 presentation:²⁵¹

Fifty milliwatts is 0.05 watts. The licensed tier's 250 mW in the UHF TV band is five times that, which is one reason large productions bother with the license.

The sources disagree slightly on when unlicensed mics arrived. Wikipedia says a January 15, 2010 order let mics in the core TV band operate "up to 50 mW without a license" under a waiver of Part 15.²⁵² The FCC's own topic page says unlicensed mics have been allowed in the TV bands under rules adopted in 2015.²⁵⁰ Both can be true - a waiver first, formal rules later - but the FCC page is the one to trust on the current rules.

What frequencies can a wireless mic use in the US today?

What you see: A frequency readout on a receiver rack, often a number between 470 and 608 MHz.

According to the FCC's wireless microphone page, usable spectrum is:²⁵⁰

• TV channels 2-36, everything below 608 MHz, on channels no local station is using
• 614-616 MHz, unlicensed (part of the 600 MHz guard band, a buffer slice that keeps neighboring services apart)
• 653-657 MHz, licensed, and 657-663 MHz, unlicensed (the duplex gap, the unpaired slice between mobile uplink and downlink)
• Licensed bands outside TV: 169-172 MHz, parts of 900 MHz, 1435-1525 MHz, and 6875-7125 MHz
• Unlicensed options outside TV: 902-928 MHz, 1920-1930 MHz, and parts of 2.4 GHz and 5 GHz

The TV-band list moves from city to city, because which channels are "unused" depends on which stations broadcast nearby. A mic that works cleanly in one town can land on top of a TV station in the next.

What happened to 700 MHz wireless microphones?

What you see: An older mic system with a label in the 698-806 MHz range, often sold cheap secondhand.

The first eviction came in 2010. The FCC prohibited wireless mics in 698-806 MHz under FCC 10-16, "Revised Rules for LPAS stations operating in the 698-806 MHz Band."²⁵⁰ The Report and Order was issued January 15, 2010, and "no longer allows Broadcast Auxiliary Service devices to operate in the 698-806 MHz portion."²⁵² The band went to mobile broadband and public safety.

The FCC set June 12, 2010 as the deadline: users of 700 MHz mics had to "retune or replace their equipment" and "cease operations" by that date.²⁵³

Is my 600 MHz wireless mic illegal?

What you see: A receiver that tunes somewhere between 617 and 698 MHz.

Probably, in the US. The second eviction followed the FCC's broadcast incentive auction, which ended April 13, 2017 and started a transition period of up to 39 months that ended July 13, 2020.²⁵⁴ Mics on 617-652 MHz and 663-698 MHz had to stop "no later than July 13, 2020," and making, importing and selling such mics is now prohibited.²⁵⁴ In places where a 600 MHz wireless carrier switched on earlier, mics had to stop earlier, "in any areas where a 600 MHz wireless licensee has commenced their operation."²⁵⁵

The FCC phased it in. From January 13, 2018, no new 600 MHz-capable LPAS devices could be certified; from October 13, 2018, legacy 600 MHz-capable Part 74 devices could no longer be sold.²⁵¹

The industry pushed back on the details. Sennheiser filed a petition for reconsideration in December 2015.²⁵⁶ In a 2017 Order on Reconsideration, the FCC let legacy 600 MHz gear be modified rather than scrapped and adopted an ETSI-based emission standard; Sennheiser called it "a victory for customers" (July 14, 2017).²⁵⁶ Joe Ciaudelli, Sennheiser's Director of U.S. Spectrum Affairs, called it "a very positive outcome for our customers."²⁵⁶ The same order optimized 169-172 MHz for more mics and clarified that the 30 MHz limit in 1435-1525 MHz applies per operator.²⁵⁶

The surviving slivers inside the old 600 MHz band, the guard band and duplex gap, are where the 20 mW limits in the table above come from.

Who coordinates frequencies at the Super Bowl?

What you see: A broadcast compound full of trucks, referees with mics, coaches with headsets, and a person with a spectrum analyzer and a very long spreadsheet.

At a big event, a frequency coordinator gives every wireless mic, IEM, intercom and radio its own frequency so nothing collides. The reason is not only crowding. When two or more transmitters mix, they create unwanted extra signals called intermodulation products, so frequencies have to be chosen as a set, not one at a time.

For football, this work is done through the Society of Broadcast Engineers (SBE) and the NFL's Game-Day Frequency Coordination program, which began with the Super Bowl: before Super Bowl XXX, NFL Films asked SBE coordinator Karl Voss to take part.²⁵⁷ Each NFL venue gets a volunteer coordinator, and a game-day database "can contain hundreds of frequencies," covering referee mics, IFBs (cueing earpieces for on-air talent), coach-to-quarterback links and security radios.²⁵⁷ Enforcement rests on voluntary cooperation backed by the threat of pulling credentials; SBE's formal authority covers only Part 74 broadcast frequencies.²⁵⁷

The numbers keep climbing:

Large events can ask for more room than the standard rules allow. Sennheiser's coverage of the 2017 order notes that operators needing more than 30 MHz should seek a special temporary authority (STA), and venues wanting 941.5-944 MHz should consult the local SBE coordinator first.²⁵⁶

What about wireless lighting control?

What you see: A small box with a stubby antenna on a lighting truss, or a battery-powered uplight with no cable running to it.

That is usually wireless DMX, the lighting-control signal sent by radio, and it commonly rides the unlicensed 2.4 GHz band alongside Wi-Fi and Bluetooth. The FCC also lists parts of 2.4 GHz among the unlicensed options for wireless mics, which says something about how many things share that band.²⁵⁰ The details of lighting control and show networks belong to the sister guide: The Show File: show networks.

Where do the mic makers come in?

What you see: A frequency chart in a mic's manual, or a web tool asking for your ZIP code.

The big manufacturers publish band charts by country because mic bands differ by jurisdiction, and both Sennheiser and Shure were active at the FCC during the 600 MHz transition. Sennheiser's petition and its reaction to the 2017 order are covered above.²⁵⁶

Shure's own guidance explains that two carriers generate third-order intermodulation products at (2 x f1) - f2 and (2 x f2) - f1, and notes that "most manufacturers recommend a minimum margin of 250kHz" between any such product and a tuned carrier.²⁶¹ Shure tells users to scan for open frequencies with the receiver, and calls coordination software such as its free Wireless Workbench "by far the best method, especially when using multiple wireless systems."²⁶² It also warns that "the environment changes constantly," so a fixed installation still needs rechecking.²⁶²

In Boston

The FCC's rule that mics may use only TV channels no local station occupies means the usable list in Boston is set by Boston's TV stations, not by the national table.²⁵⁰ Anyone running a show in the city works from that local list, the 600 MHz slivers, and the bands outside TV.

On the football side, the SBE/NFL model assigns a volunteer coordinator to each NFL venue.²⁵⁷

Elsewhere in the world

The US model is not universal. According to Wikipedia's survey of wireless mic licensing:

• UK: Ofcom licenses wireless mics as PMSE (programme making and special events). Channel 38 (606-614 MHz) is licensed "on a shared basis," and users coordinate among themselves.²⁵²
• Australia: a class license from ACMA allows mics up to 100 mW EIRP in 520-694 MHz on unused TV channels, with no individual license needed.²⁵²
• Europe: licensing is coordinated through CEPT and its Electronic Communications Committee (ECC), and "in many other countries" mics require a license.²⁵²

A US-market mic carried abroad may be illegal there, and the reverse is also true. The UK and EU have had their own digital-dividend clearances on their own timetables, and ISED in Canada and MIC in Japan set their own rules, so check the national regulator before taking a system abroad.

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15Hams, SDRs and number stations

The short version: Amateur radio ("ham" radio) is a licensed, non-commercial radio service, and in the US you earn transmitting privileges by passing a written multiple-choice exam: Technician first, then General, then Amateur Extra. The UK runs a parallel ladder of Foundation, Intermediate and Full licenses, updated by Ofcom in December 2023. Listening is a separate hobby: a USB dongle costing about $30 can receive aircraft position broadcasts, FM radio and much more, though what you may legally listen to and share varies by country (and, in the US, by state). And on 4625 kHz, a Russian station has been buzzing since at least 1982 for reasons nobody official has explained.

What is a ham, and how do you become one in the US?

What you see: A car with a tall whip antenna and a license plate that looks like gibberish (W1-something), or a wire strung between two trees in someone's yard.

Amateur radio is a licensed radio service for hobby, experimentation and emergency communication, with no commercial traffic. In the US, licenses are issued by the FCC, and the three classes issued today each require a written exam. Federal rules (47 CFR 97.503) set the exams:²⁶³

Each class builds on the one below and opens more frequencies and privileges. Two older classes, Advanced and Novice, are no longer issued, though holders of those classes still appear in the FCC database.²⁶⁴

Every licensed station has a call sign, a unique identifier it must transmit, such as W1MX. That vanity-looking plate on the car is usually the owner's call sign.

In the US, the FCC issues call signs in sequence from groups tied to license class. Amateur Extra holders (Group A) can get the shortest forms, such as one letter, a digit and two letters (1x2, like W1MX); General and Technician holders (Group C) get one letter, a digit and three letters (1x3); Novice and club stations (Group D) get two letters, a digit and three letters (2x3). The digit marks the region where the call was first issued, and region 1 covers Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island and Vermont; under the vanity program a licensee may list up to 25 preferred call signs, but only from groups their class is eligible for.²⁶⁵

A US amateur license "is normally granted for a 10-year term,"²⁶⁶ and since April 19, 2022 the FCC has charged $35 per application for new licenses, renewals, rule waivers and vanity call sign requests, while upgrades, sequential call sign changes and address updates are free.²⁶⁷

How many ham radio operators are there?

What you see: A headline claiming ham radio is dying, or one claiming it is booming.

The last full class-by-class breakdown cited here is the ARRL's count of the FCC database as of December 31, 2016: 742,787 licensees.²⁶⁴

That year 32,552 new licenses were issued.²⁶⁴ For 2017, blogger and instructor KB6NU reported 748,136 licensees in the FCC database, citing the ARRL for that year's 32,196 new licensees.²⁶⁸ Wikipedia, citing the ARRL, says more than 30,000 new US licenses were issued in 2023.²⁶⁹

The ARRL (American Radio Relay League), the national amateur radio association, was co-founded on April 6, 1914, by Hiram Percy Maxim and Clarence D. Tuska of Hartford, Connecticut, and is headquartered in Newington, Connecticut.²⁶⁹

What are the UK amateur radio licence levels?

What you see: A UK call sign beginning M7, M8, M9 or M0 on a QSL card, an online log or a club website.

Ofcom keeps a three-step ladder and updated it in a statement published December 11, 2023, with changes phased in from an expected February 2024 license variation through the 2024/25 financial year.²⁷⁰ The power limits, given as PEP (peak envelope power, a way of measuring transmitter output), are:²⁷⁰

UK licenses remain lifetime, and a person may hold one personal license and call sign.²⁷⁰ Ofcom said there were "more than 100,000 amateur radio licences in the UK currently" as of December 2023.²⁷⁰

One visible feature of the UK system is its power tiers: the entry license comes with a stated 25 W ceiling in most bands. US rules work differently: every amateur station must use "the minimum transmitter power necessary," with an overall ceiling of 1.5 kW PEP, and Novice and Technician operators are held to 200 W PEP in the HF segments open to them.²⁷¹

Ofcom implemented Phases 2 and 3 of its review together on October 24, 2025, after earlier targets of December 2024 and March 2025 slipped.²⁷² Since then, UK licensees can propose a new call sign of their choice through Ofcom's online portal once every five years, and a surrendered or revoked call sign stays unavailable for five years.²⁷²

Are there ham radio clubs in Boston?

What you see: A flyer for a "swapfest," or a group of people at a folding table with radios at a public event.

Yes, and two are easy to find.

The MIT Radio Society, W1MX (formerly 1XM), calls itself "America's oldest college amateur station." Its station is in Room 50-358 in Walker Memorial. It holds open meetings on Fridays at 7:00 pm, with no experience required, and runs the MIT Swapfest, a radio and electronics flea market.²⁷³

The Boston Amateur Radio Club, W1BOS, meets on the third Thursday of the month at 7:30 pm at Artisans Asylum, 96 Holton St in Allston, with no meetings in July or August. It runs a VHF analog repeater and a UHF DMR (digital) repeater downtown.²⁷⁴ A repeater is a station, usually on a tall building or hill, that receives a weak signal and re-transmits it at higher power, so two handheld radios across the city can reach each other.

What is a software-defined radio, and what is an RTL-SDR?

What you see: A USB stick the size of a thumb drive with a coax antenna lead, plugged into a laptop showing a scrolling rainbow "waterfall."

A software-defined radio (SDR) does its tuning and demodulation in software instead of in dedicated circuits. The cheapest way in is the RTL-SDR: a USB TV-tuner dongle built on the Realtek RTL2832U chip. RTL-SDR.com credits Antti Palosaari, Eric Fry and the Osmocom project (especially Steve Markgraf) with discovering that "the raw I/Q data on the RTL2832U chipset could be accessed directly," which turned a TV stick into a general-purpose receiver.²⁷⁵

According to RTL-SDR.com, a typical dongle costs "~$30" and covers roughly 500 kHz to 1.75 GHz depending on model, with about 24-1766 MHz for the common R820T, R860 and R828D tuners.²⁷⁵ These are receivers. They do not transmit.

The waterfall on screen is a live picture of the spectrum: frequency across, time down, signal strength as color. Every bright stripe is someone transmitting.

What can I listen to with an RTL-SDR?

What you see: A map on a laptop with little airplane icons moving across it, or a grainy black-and-white satellite picture of the coastline.

RTL-SDR.com lists these as common uses:²⁷⁵

• ADS-B aircraft tracking: aircraft broadcast their own position, and a dongle can plot them on a map
• Weather-satellite images from NOAA satellites (no longer possible; see below)
• FM broadcast radio, including the RDS text that shows station names
• Aviation voice scanning
• AIS ship tracking

In Boston, aircraft near Logan Airport (BOS) are the obvious ADS-B target for anyone curious about what is flying over; aviation explains what those broadcasts contain.

The NOAA satellite pictures are now history for NOAA's own spacecraft. NOAA decommissioned NOAA-18 on June 6, 2025, after an unrecoverable transmitter failure, then NOAA-19 on August 13, 2025, and NOAA-15 on August 19, 2025.¹⁸³ What is still overhead is in satellites.

For police, fire and EMS radio, including why much of it is now digital or encrypted, see public safety and scanners; how dispatch works is in the sister guide How a City Works.

Is it legal to listen?

What you see: A forum argument about whether listening to something is a crime.

Broadcast stations are meant to be heard by the public. Beyond that, the law turns on what is being received and what you do with it, and it differs by country and, in the US, by state. This guide does not cover how to receive private or encrypted communications, and nothing here should be read as permission to do so.

What follows describes the main statutes; it is not legal advice.

In the US, the Electronic Communications Privacy Act makes it a federal offense to intentionally intercept an electronic communication, or to disclose its contents knowing it was unlawfully intercepted.¹³⁹ The same law exempts several kinds of radio traffic, including stations transmitting for the general public, distress traffic, amateur, CB and GMRS stations, marine and aeronautical systems, and government, police and public-safety systems that are "readily accessible to the general public."¹³⁹ A radio communication does not count as readily accessible if it is "scrambled or encrypted" or carried over a common carrier's system, such as a phone network, among other cases.¹¹¹

Separately, 47 USC 605 bars anyone not authorized by the sender from intercepting a radio communication and divulging or publishing its contents, with exceptions for broadcasts meant for the public, distress traffic, and amateur and CB transmissions.²⁷⁶

In the UK, section 48 of the Wireless Telegraphy Act 2006 makes it an offense, without lawful authority, to use radio apparatus to obtain information about the contents, sender or addressee of a message the user is not an intended recipient of, or to disclose such information.¹²⁰ The disclosure offense applies only to information that came from radio apparatus, does not cover disclosure in legal proceedings, and carries a fine of up to level 5 on summary conviction.¹²⁰

If you are unsure, the national regulator (the FCC in the US, Ofcom in the UK, ISED in Canada, ACMA in Australia, MIC in Japan) and the national amateur radio society are the places to check.

What is UVB-76, "The Buzzer"?

What you see: A YouTube clip of a monotonous buzz, captioned with a spooky story about Cold War doomsday machines.

A number station is a shortwave station broadcasting coded numbers or tones of unknown purpose. The most famous is UVB-76. Every fact below comes from Wikipedia, so treat this as a fun fact rather than intelligence.

According to Wikipedia:²⁷⁷

• It broadcasts on 4625 kHz shortwave, and its nickname is "the Buzzer" (Zhuzhzhalka).
• The earliest preserved recording dates from 1982; reports place its start in the late 1970s, or 1976.
• The buzz repeats about 21-34 times per minute. Occasionally a voice reads code words and numbers in Russian, in message formats called Monolith and Uzor.
• The call sign "UVB-76" is a mistranscription of "UZB-76."
• It is now thought to transmit from near Naro-Fominsk, near Moscow.
• The popular theory that it is part of the "Dead Hand" nuclear doomsday system is called unlikely.

Its purpose has never been officially confirmed. Shortwave signals bounce off the ionosphere, which is why a Russian station can be heard far beyond Russia, especially at night.

Elsewhere in the world

The US and UK are the two systems covered here in detail, and the UK's stands out for its explicit power tiers and lifetime licenses.²⁷⁰ ISED in Canada, ACMA in Australia and MIC in Japan run their own amateur licensing, so check the national regulator for exams, call signs and power limits. Receiving rules differ from country to country too.

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16Is all this radio safe? What the agencies say

The short version: Governments cap radio-frequency (RF) exposure with limits designed to prevent tissue heating, with safety margins built in. The US uses FCC limits dating to 1996; an international commission, ICNIRP, publishes guidelines that were updated in 2020 to cover 5G. In 2011 the WHO's cancer agency, IARC, classified RF fields as "possibly carcinogenic" (Group 2B) on limited evidence, and a WHO-commissioned review published in 2024 found moderate-certainty evidence that mobile phone use likely does not raise the risk of brain and other head tumors, a finding some critics dispute. This section reports what each body says, with attribution, and does not offer advice.

What does "radiation" mean here?

What you see: A phone's legal screen listing a "SAR" figure, or a sticker on a cell site warning about RF exposure.

Radio waves, Wi-Fi, cell signals and microwaves are non-ionizing radiation: they do not carry enough energy per photon to knock electrons off atoms, which is what X-rays and nuclear radiation do. The question agencies study is what lower-energy RF does to the body.

The World Health Organization's answer on the known mechanism: "Tissue heating is the main mechanism of interaction between radiofrequency fields and the human body," and current exposure levels cause a negligible temperature rise.²⁷⁸ Exposure limits are built around that mechanism.

The key unit is SAR (specific absorption rate): how much RF energy body tissue absorbs, measured in watts per kilogram (W/kg).

What is the SAR limit for cell phones in the US?

What you see: "SAR: 1.19 W/kg" or similar in a phone's settings or manual.

The FCC's limits for the general public are in 47 CFR 1.1310. For 100 kHz to 6 GHz they are:²⁷⁹

For far-field exposure, the same rule sets a general-population power-density limit of 1.0 mW/cm2 from 1,500 to 100,000 MHz.²⁷⁹

These limits have a history. The FCC adopted exposure limits based on recommendations from the National Council on Radiation Protection and Measurements (NCRP) on August 1, 1996, covering 300 kHz to 100 GHz. Device SAR limits took effect August 7, 1996, and all licensees had to comply by September 1, 2000.²⁸⁰ The FCC's technical guide for testing compliance is OET Bulletin 65, "Evaluating Compliance With FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields."²⁸⁰

The FCC's RF safety page says it "is confident in its RF exposure guidelines," and that in 2019 it concluded the best available scientific evidence supports maintaining the existing limits.²⁸¹

In the US, the FDA shares regulatory responsibility for cell phones with the FCC: the FCC sets the emission limits and the FDA provides scientific input.²⁸² The FCC describes the FDA as the agency with "primary jurisdiction for investigating mobile phone safety."²⁸³ In 2019, Jeffrey Shuren, director of the FDA's Center for Devices and Radiological Health, wrote to the FCC that "the existing safety limits for cell phones remain acceptable for protecting the public health."²⁸⁴ The FCC adopted its decision to keep its limits (FCC 19-126, ET Docket 19-226) on November 27, 2019, and released it on December 4, 2019.²⁸⁵²⁸⁶

What are the ICNIRP guidelines?

What you see: A European or UK phone spec sheet quoting a SAR figure "over 10 g," or a council report citing "ICNIRP compliance" for a new mast.

ICNIRP, the International Commission on Non-Ionizing Radiation Protection, publishes the exposure guidelines used by many countries. Its current RF guidelines, covering 100 kHz to 300 GHz, were published in March 2020 in the journal Health Physics. They replace the 1998 RF guidelines and, in ICNIRP's words, "cover many applications such as 5G technologies, WiFi, Bluetooth, mobile phones, and base stations."²⁸⁷

The general-public basic restrictions in the 2020 guidelines are:²⁸⁸

For the general public, ICNIRP applies a reduction factor of 50 for whole-body exposure and 10 for local limits; these are its safety margins.²⁸⁸ Above 6 GHz, energy is absorbed close to the skin surface, which is why the guidelines switch from SAR to a power-density measure.

Why is the US limit 1.6 and Europe's 2.0?

What you see: Two different SAR numbers for the same phone model in different countries.

The two figures are measured differently. The FCC averages over 1 gram of tissue;²⁷⁹ ICNIRP averages over 10 grams.²⁸⁸ Averaging over a smaller mass captures sharper hot spots, so a 1 g number and a 10 g number for the same phone are not directly comparable, and "1.6 is stricter than 2.0" is not a fair reading on its own. The whole-body limit, 0.08 W/kg, is the same in both.²⁷⁹²⁸⁸

Canada's limits are set in Health Canada's Safety Code 6 (2015). For the general public they are 0.08 W/kg whole-body, 1.6 W/kg over any 1 g for the head, neck and trunk, and 4 W/kg over 10 g for limbs, averaged over 6 minutes, so Canada uses the same 1 g measure as the US.²⁸⁹

Switzerland applies a precautionary principle under which, according to its Federal Office of Communications, radiation limits "must be ten times lower in Switzerland than in neighbouring countries."²⁹⁰ Italy long used a 6 V/m attention value and quality objective for RF fields; according to the Molise regional environmental agency (ARPA Molise), Law 214 of December 30, 2023 raised both to 15 V/m, fully in effect from April 2024.²⁹¹

What does "possibly carcinogenic" (Group 2B) mean?

What you see: A headline saying "WHO: cell phones may cause cancer."

On May 31, 2011, in Lyon, a working group of 31 scientists from 14 countries convened by the International Agency for Research on Cancer (IARC), part of the WHO, classified RF electromagnetic fields as Group 2B, "possibly carcinogenic to humans."²⁹²

According to IARC's press release:²⁹²

• The basis was "limited" evidence for glioma (a brain tumor) and acoustic neuroma (a tumor of the hearing nerve) in wireless phone users, and "inadequate" evidence for other cancers.
• One study of past cell phone use (up to 2004) found a 40% increased glioma risk in the highest category of heavy users, who reported an average of 30 minutes a day over 10 years.
• Working group chair Jonathan Samet said: "The conclusion means that there could be some risk."
• IARC adds that the words "probably" and "possibly" carcinogenic "have no quantitative significance."

That last point matters for reading headlines. IARC's groups describe how strong the evidence is that something can cause cancer, not how large any risk is.

Australia's radiation agency, ARPANSA, describes the IARC classification as "largely based on limited evidence from human observational studies," and dates it to 2013,²⁹³ the year the full monograph was published, while IARC's own press release dates the decision to May 2011.²⁹² Both dates appear in coverage; they refer to the meeting and the publication.

In 2024, an IARC Advisory Group recommended, in a summary published in The Lancet Oncology, that RF fields be re-evaluated "with high priority" in the latter half of 2025-2029.²⁹⁴ IARC's announcement of its next listed meeting, Volume 144 on lead and cadmium compounds (October 19-26, 2027), does not mention RF,²⁹⁵ and Microwave News reported in 2024 that no date for an RF review had been set.²⁹⁶

What did the 2024 WHO review find about phones and brain cancer?

What you see: A 2024 headline saying "WHO review finds no link between mobile phones and brain cancer."

WHO surveyed experts in 2018 to set research priorities and then commissioned a series of systematic reviews (studies that gather and grade all the relevant research on a question by a preset method).²⁹⁷ The cancer review in humans was led by Ken Karipidis of ARPANSA and published in Environment International in 2024:

Karipidis K, Baaken D, Loney T, Blettner M, Brzozek C, Elwood M, Narh C, Orsini N, Röösli M, Paulo MS, Lagorio S. "The effect of exposure to radiofrequency fields on cancer risk in the general and working population: A systematic review of human observational studies - Part I: Most researched outcomes." Environment International 2024; 191:108983.²⁹⁷

What it reported:²⁹⁷

ARPANSA, announcing the review on September 4, 2024, said more than 5,000 studies from 1994-2022 were screened and 63 were included, and that there has been "no rise in the incidence of brain cancers" despite greatly increased wireless use.²⁹³ ARPANSA's summary: "we can be more confident that exposure to radio waves from wireless technology is not a human health hazard."²⁹³ It also said WHO is preparing an Environmental Health Criteria monograph that will draw on this and the other commissioned reviews.²⁹³

The review has critics. Microwave News, a newsletter on non-ionizing radiation, wrote in September 2024 that "the results of this review were never in doubt," pointing to the authors' ties to ICNIRP, the inclusion of a Danish cohort study not designed to study cell phones, and the omission of rodent studies by the US National Toxicology Program and the Ramazzini Institute.²⁹⁸ The same article quotes the International Commission on the Biological Effects of Electromagnetic Fields (ICBE-EMF): "There is no scientific justification for concluding there is any certainty that RF exposures do not cause cancer."²⁹⁸

In a January 2025 reply in Environment International to a critical letter by Di Ciaula and colleagues, the review's authors said the review covered only human studies and that a separate review of animal studies was in progress.²⁹⁹

As of October 2026, WHO's radiofrequency page still describes the Environmental Health Criteria monograph as work under way, a health risk assessment "to be published as a monograph," with no publication date given.³⁰⁰

Is 5G dangerous to health?

What you see: A flyer or post linking 5G masts to illness, or the 2020 claims linking 5G to COVID-19.

The WHO's Q&A on 5G, dated February 27, 2020, says:²⁷⁸

• "No adverse health effect has been causally linked with exposure to wireless technologies."
• 5G at around 3.5 GHz produces exposure "similar to that from existing mobile phone base stations."
• Beam-forming, where 5G antennas steer energy toward users, may make exposure more variable, and this is still being studied.
• "Only a few studies" have examined the frequencies 5G uses, so conclusions draw on research across the wider spectrum.

ICNIRP states that its 2020 guidelines explicitly cover 5G.²⁸⁷ The WHO Q&A predates the 2024 review described above. How 5G networks are built is covered in cellular.

Some local governments and campaign groups have called for 5G moratoriums; this section quotes only national and international agencies. For what the boxes on poles and lampposts are, see the sister guide Street Code: utility poles.

In Boston

RF exposure limits in the US are set federally by the FCC, and the limits above are the ones that apply to devices and transmitters in Boston.²⁷⁹

Federal law also limits what states and cities can do. Under 47 USC 332(c)(7)(B)(iv), no state or local government may regulate the placement, construction or modification of personal wireless service facilities "on the basis of the environmental effects of radio frequency emissions," as long as those facilities comply with the FCC's rules.³⁰¹ For where the area's large transmitters stand, see towers.

Elsewhere in the world

ICNIRP's guidelines are international and explicitly cover phones, Wi-Fi, Bluetooth, base stations and 5G;²⁸⁷ the US instead sets its own limits through the FCC.²⁷⁹ The practical difference most people meet is the SAR figure on a phone: 1.6 W/kg over 1 g under US rules, 2 W/kg over 10 g under ICNIRP. IARC's 2011 classification is a single global finding, while national agencies differ in how they explain it to the public, as ARPANSA's framing above shows.²⁹³

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17The airwaves over Boston

The short version: Most of the big signals over Boston do not come from downtown. TV comes from a cluster of tall towers on the Needham/Newton line near Route 128, much of Boston's FM has long come from a mast on top of the Prudential, and WBZ 1030 AM comes from two towers on the water in Hull. Underground, the T carries cell signals over fiber and leaky cable, while transit and city crews are moving their own radios to the P25 digital standard. Planes, ships and now electric meters announce themselves by radio too, and the region's radio history includes two famous firsts, one of them disputed.

Where do Boston TV stations broadcast from?

What you see: A cluster of very tall towers on the Needham/Newton line near Route 128.

Boston TV signals come from Needham. WBZ-TV transmits from Cedar Street there, at about 42°18′37″N 71°14′12″W.³⁰² The tower went up in 1957 to replace an original tower destroyed in 1954; it stood about 1,200 ft (366 m) at first and was described as a 1,295 ft (395 m) guyed tower in 2009, according to the trade historian Scott Fybush.³⁰³ The site was formerly owned by CBS and now belongs to American Tower.³⁰²

WBZ-TV is on RF channel 20, virtual channel 4, and it broadcast on analog VHF channel 4 from 1948 to 2009.³⁰² The virtual channel is the number your TV shows; the RF channel is the frequency the signal actually uses. WCVB became a tenant at Cedar Street in 1972, and other tenants include WSBK, WBTS-CD and WGBX.³⁰³³⁰²

Radio lives here too. The nearby "FM-128" community tower was formerly WHDH-TV's channel 5 tower, and WCRB 102.5 moved there.³⁰³ WBZ-FM 106.7 once transmitted from Cedar Street, left in 1981, and that frequency is now WMJX at the Pru.³⁰³

From downtown, Needham lies to the west-southwest, which is the direction a city rooftop TV antenna generally faces.

What is the stainless-steel tower with arms in Needham?

What you see: A tall tower whose top splits into several arms, each holding its own antenna, like a candle holder.

That is the Cabot Street candelabra, which Fybush describes as a "stainless candelabra tower that went up at the end of the 1960s," built out in the early 1970s, with American Tower as landlord.⁵⁷ In 2009 it carried WFXT (channel 25), WSBK (38) and WLVI (56).³⁰³

The repack was the FCC's reshuffle of TV channels after it auctioned part of the TV band to wireless carriers. After the repack, WGBH and WFXT made Cabot Street their main site, and the candelabra became the full-power backup for WBZ-TV, WCVB, WSBK and WGBX.⁵⁷

WGBH-TV (PBS, virtual channel 2) now transmits on low-band VHF, RF channel 5, with its antenna at the very top of the stack.⁵⁷ WBTS-CD, the NBC station branded NBC 10, shares WGBX's RF channel 32.⁵⁷

What antennas are on top of the Prudential?

What you see: A slim mast rising from the roof of the Prudential Tower in the Back Bay.

The Pru carries FM, not big TV. As of 2004, its mast rose 157 ft (48 m) above the roof to 907 ft (276 m) above ground, measured to the tip of an old channel 68 TV antenna.³⁰⁴ A six-bay panel "master" antenna, shared by several stations, carried 100.7, 104.1, 106.7 and 107.9 MHz, and a separate ERI panel antenna carried Greater Media's 92.9, 96.9 and 105.7, which had moved there from the FM-128 tower.³⁰⁴ A shared master antenna lets several stations combine their signals into one antenna instead of each hanging its own.

The John Hancock Tower is about 40 ft (12 m) taller than the Pru, but in 2004 its only broadcast tenant was a small FM translator, W267AI.³⁰⁴

Those lists are more than 20 years old, and call letters and formats have changed since. The FCC's FM Query database shows current licensees.

Where is WBZ's transmitter?

What you see: Two tall radio towers by the water in Hull.

WBZ 1030 transmits from Hull. Fybush reports the site was dedicated on July 8, 1940, with two 500 ft towers (152 m), each a half wavelength tall at WBZ's old frequency of 990 kHz and spaced a quarter wavelength apart.³⁰⁵ Fed together, they form a directional array with a heart-shaped ("cardioid") pattern that cuts the signal over the Atlantic and throws it toward Boston.³⁰⁵ Salt water lies between the towers and the city.³⁰⁵ AM ground signals travel well over salt water.

The sources disagree on details:

Sources: Wikipedia³⁰⁶ and Fybush.³⁰⁵ The two dates may describe different events (a dedication and a move), but neither source says so. The FCC's tower registration would settle the height.

Why can I hear WBZ 1030 at night in other states?

What you see: WBZ coming in clearly on a car radio hundreds of miles from Boston after dark.

WBZ is a Class A clear-channel station.³⁰⁶ A clear channel is an AM frequency where one high-power station is protected from interference over a wide area, day and night. WBZ runs 50,000 watts around the clock.³⁰⁵ At night, AM signals bounce off the upper atmosphere (skywave) and come down far away. Fybush reported in 2005 that WBZ reached New York City strongly at night, while the Outer Cape got a weaker signal, and that WBZ and WWL in New Orleans were the only directional I-A clear channels still operating that way.³⁰⁵

The station is one of the oldest in the US. It was licensed on September 15, 1921, and first broadcast on September 19 from the Eastern States Exposition, operating from Westinghouse in East Springfield.³⁰⁶ It opened a Boston studio in 1924, was assigned 990 kHz in 1928, moved its transmitter to Millis in 1931 and reached 50 kW in 1933.³⁰⁶ On March 29, 1941, it moved to 1030 kHz under the North American Regional Broadcasting Agreement (NARBA).³⁰⁶ iHeartMedia has owned it since December 19, 2017.³⁰⁶

How does cell service reach the T tunnels?

What you see: Small antennas and cable runs along subway tunnel walls and station ceilings, and a phone that works underground.

Your phone cannot hear a street-level tower through the ground, so the T uses a distributed antenna system (DAS), a common cellular fix for tunnels and large buildings. Carrier signals are converted to light, carried over fiber, then turned back into radio at remote amplifiers in the tunnels.⁷³ Much of the signal leaves through radiating cable (also called leaky feeder), coaxial cable with slots that let radio leak out along its length.

The MBTA signed a 15-year contract with InSite Wireless in 2005, and InSite pays the T a revenue share, about $5.3 million expected over the term.⁷³ Service arrived in stages:

InSite runs the shared network, and each carrier connects at a head-end. The build included 71 remote amplifier sites, 354 antennas and 41,585 ft (12.7 km) of radiating cable.³⁰⁷

The size depends on who is counting. The Boston Globe in 2012 described 19 miles of tunnel and 35 underground stations.⁷³ The Wireless Infrastructure Association, in an article dated June 23, 2026, says the DAS was first deployed in 2005 and covers more than 20 miles of tunnel and more than 40 stations, and that American Tower worked with the MBTA to replace legacy equipment and enable 5G.⁷⁴

What radio do MBTA crews and Transit Police use?

What you see: Handheld radios on train operators, station staff and Transit Police officers.

On December 16, 2022, L3Harris announced a five-year, $93 million MBTA contract to replace the legacy Enhanced Digital Access Communications System (EDACS) with P25, the North American digital standard for public safety radio.³⁰⁸ The company said the work would give more users access to the system, including MBTA police, and improve reliability.³⁰⁸ How P25 works is covered in public safety, and dispatch in the sister guide How a City Works.

L3Harris's announcement does not give the band the new system uses, a completion date, or how it covers the tunnels.

What radio system do Boston police and firefighters use?

What you see: Officers' shoulder microphones, and antennas on fire stations and police buildings.

Boston Police, Fire and EMS radio runs on UHF. Introba, the consultant on the city's upgrade, describes "P25 digital and analog" service and new UHF frequencies for interoperability.³⁰⁹ In 2024 the city put out to bid a Radio Upgrade Project (EV00014215, through the Fire Commissioner) covering ten communication sites, with a term from July 1, 2024 to June 30, 2027; the bid pages do not show an awardee.³¹⁰³¹¹

On August 18, 2025, a Boston Police radio channel failed during a night of shootings, weeks after the new system was adopted.³¹² The accounts differ: the police union said the outage lasted about 30 minutes, while the department called it "very short" and said it affected one channel.³¹² NBC Boston reported that the department was working with Motorola on the cause and that city councilors sought a hearing.³¹²

Boston Police radio audio is now encrypted. On August 9, 2025, the department switched from analog to an encrypted digital system and said its transmissions would "no longer be available via traditional analog radio scanning equipment."⁹⁰ The department said encryption would prevent interference and monitoring by "bad actors," and offered a free, delayed public audio feed through RapidSOS instead.⁹⁰ The Boston Globe reported the delay would be about 5 minutes in most cases; Justin Silverman of the New England First Amendment Coalition argued the change cuts off real-time information for journalists and the public, and suggested encrypting only sensitive channels.¹¹⁸ What scanner listeners can still hear, and the law on listening, is in scanners.

Why are planes flying over my house in Somerville, Cambridge or Milton?

What you see: A steady line of jets over the same streets, sometimes for hours, then none for days.

The runway Logan is using decides which neighborhoods hear planes, and satellite navigation has narrowed the paths. Satellite-guided RNAV procedures arrived at Logan in 2012-2013 and concentrated flights onto tighter tracks, according to MIT.³¹³

A collaboration among MIT, the FAA and Massport began in 2016 to design procedures that spread or move the noise. One result, a Runway 33L RNP approach with curved segments, came in 2021 and was estimated to put 2,954 fewer people above 60 decibels on a single flight.³¹³ An over-water RNAV approach to Runway 22L was planned for 2024, and MIT named Hull, Lynn and Salem among towns affected by arrival noise.³¹³ Cambridge opposed a 2021 proposed change to 33L departures.¹⁴⁷

Altitude depends on where you are. Milton says Runway 27 departures cross the town at about 3,500 ft (1,070 m) and arrivals pass at about 1,700-2,500 ft (520-760 m).³¹⁴

To complain or ask questions, Massport runs a Noise Abatement Office and a complaint line, 617-561-3333 (staffed 6 am to 5 pm on business days), plus an online form.¹⁴⁷ The FAA's New England noise ombudsman takes email at 9-ane-noise@faa.gov, and the state legislature created a Massport Community Advisory Committee.¹⁴⁷

How can I tell which plane just flew over Boston?

What you see: A jet overhead, and a question about where it came from.

Massport runs a Noise and Operations Monitoring System (NOMS) and a near-live public "Flight Monitor" that shows aircraft tracks around Logan, according to the City of Cambridge.¹⁴⁷ That is the official route. Consumer flight-tracking apps use the position reports aircraft broadcast by radio; how they work, and the frequencies involved, are covered in aviation.

How do I track ships in Boston Harbor?

What you see: A ship-tracking website showing a tanker or container ship moving up the harbor channel.

Large ships broadcast their identity, position and course on the Automatic Identification System (AIS), a data system on marine VHF, and ship-tracking websites collect those broadcasts (more in ships). In the US, 47 CFR 80.393 sets aside four 25 kHz simplex channels for AIS only:³¹⁵

Many of those ships also carry a local pilot. Under compulsory pilotage, a vessel may be legally required to use a state- or federally licensed pilot, depending on its type, tonnage, draft, flag, trade and the waters it is in.³¹⁶ State pilotage generally applies in state waters, and federal pilotage applies to some US-flag vessels.³¹⁶

In Massachusetts, pilotage is governed by M.G.L. c. 103 and, for the district around Boston, by the state regulations at 995 CMR 2.00, which list Lynn, Saugus, Hull, Hingham, Weymouth, Braintree and Quincy harbors.³¹⁷ The pilots of that district, District One, act jointly as the "Boston Pilots" and may operate as the Boston Harbor Pilots Association, LLC.³¹⁷ Their normal boarding area is a precautionary area with a radius of 6.17 miles centered on 42°22.71′N, 70°46.97′W, a position the pilots must have published in the US Coast Pilot.³¹⁷

What is the new meter on my house, and can I opt out?

What you see: A new electric meter with a digital display, installed by Eversource.

Eversource began replacing Massachusetts electric meters with Itron smart meters in summer 2025, under advanced metering infrastructure (AMI) plans the Department of Public Utilities approved in 2022.³¹⁸ The meters send readings by radio over a private network, allowing daily usage data, and need no Wi-Fi.³¹⁸ Eversource's AMI budget is $668 million; it reached 100,000 installs in January 2026, plans more than 1.5 million meters, and is doing denser areas near Boston later in the rollout.³¹⁸

Opting out costs $34 a month,³¹⁸ and Eversource said in 2025 it would also charge a one-time $42 fee.²³⁷ Two bills in the legislature, S.2306 and H.3551, would change the opt-out rules (as reported in February 2026).³¹⁸

On health, Eversource says the meters' RF levels are "well below national guidelines."³¹⁸ The health section covers what the FCC and health agencies say about RF exposure, and everyday radios covers both utilities' rollouts.

Eversource's smart meter FAQ (as of October 2026) says the meters have "an embedded radio" for two-way communication with the company and do not rely on Wi-Fi.³¹⁹ It gives a measured exposure of 0.06 mW/cm² at 8 inches while transmitting, at 902 MHz, which it calls almost 10 times lower than the FCC limit.³¹⁹ The FAQ does not say whether the meters use a mesh or a cellular network.

Was the first radio broadcast really from Brant Rock in 1906?

What you see: A plaque, an anniversary story, or a ham station on the air every Christmas Eve.

Reginald Fessenden's station at Brant Rock, on the Massachusetts South Shore, is often credited with the first broadcast of voice and music, on Christmas Eve 1906. The evidence is thin. The claim rests on a letter Fessenden wrote on January 29, 1932 to S. M. Kintner of Westinghouse, and as the historian Thomas White puts it, "There do not seem to be any contemporary accounts."³²⁰ Helen Fessenden's 1940 book repeats his account, which claimed reception "as far down as Norfolk, Va."³²⁰

The engineer and writer James O'Neal found that searches in 1956 turned up no newspaper or journal mention, and that Navy, United Fruit and Frank Hart logs show nothing unusual.³²¹ What is documented is a December 21, 1906 demonstration, in which the alternator produced about 36 watts and about 12 watts reached the antenna.³²¹ O'Neal suggests Fessenden may have conflated events with Navy tests in 1909, and concludes the Christmas Eve broadcast can be neither confirmed nor ruled out.³²¹

The anniversary is still marked: in 2020 WA1ZMS transmitted on 486 kHz on Christmas Eve under FCC Part 5 experimental license WI2XLQ, as it had each December for several years.³²²

Where was Marconi's station on Cape Cod?

What you see: A marker on a bluff in South Wellfleet, inside Cape Cod National Seashore.

Guglielmo Marconi arrived on Cape Cod in February 1901 and built four 210 ft (64 m) towers in South Wellfleet.³²³ The historical marker at the site says that on January 18, 1903, a 48-word message was sent "from here to England" and a reply received, "the first two-way transoceanic communication."³²⁴ President Theodore Roosevelt's greeting to King Edward VII is dated "Wellfleet Mass January 19 1903."³²⁴ Cape Cod & the Islands magazine dates the Roosevelt exchange to January 18, so "January 18-19, 1903" covers both accounts.³²³

The station relayed the distress call of the steamship Republic in 1909. The Navy shut it in 1917, it was dismantled in 1920, and the site is now part of Cape Cod National Seashore.³²³

Elsewhere in the world

Clustering TV towers on high ground outside downtown is a common pattern: Mount Wilson above Los Angeles and Sutro Tower in San Francisco are classic examples, alongside Needham.³²⁵ WBZ's place on the dial was set under NARBA, a North American agreement, so its clear-channel status is a North American arrangement.³⁰⁶ The AIS table above quotes the US rule, 47 CFR 80.393;³¹⁵ AIS itself is a system standardized by the International Telecommunication Union (ITU), according to the US Coast Guard.³²⁶ P25, the standard Boston and the MBTA are adopting, is a North American standard; much of Europe uses a different one, TETRA, an ETSI standard that national police, fire and ambulance networks such as Belgium's ASTRID, the Netherlands' C2000 and Sweden's RAKEL run on.³²⁷

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18Reading an antenna: towers, farms and the public records behind them

The short version: Tall transmitting towers tend to gather in clusters called antenna farms, usually on high ground and often outside the city center. In the US, anyone can look up registered towers for free in the FCC's Antenna Structure Registration (ASR) database, and look up wireless licenses in the Universal Licensing System (ULS); broadcast stations have their own Media Bureau search tools, and regulators in Canada, the UK and Australia run similar public registers. The records cover tall structures and licensed transmitters, not every small antenna, and the shape of a structure tells you something about what it does.

What is an antenna farm?

What you see: Several tall towers standing close together on a hill or beside a highway, miles from the city center.

An antenna farm is a place where many transmitting or receiving antennas, towers or dishes are clustered, usually on high ground.³²⁵ Stations go there for height, to share costs, for backup (one tower can cover for another), and because high ground has fewer obstructions.³²⁵ Wikipedia's examples include Dedham/Needham outside Boston, Mount Wilson above Los Angeles and Sutro Tower in San Francisco.³²⁵

The term has an odd legal history in the US. The FCC's rules contain a section titled "Designated Antenna Farm Areas," 47 CFR 17.9, but it has been a placeholder for decades: the FCC has never designated or defined an antenna farm.³²⁸ Its environmental rules do refer to de facto areas "in which similar antenna towers are clustered."³²⁸ So the antenna farms you see grew by habit and economics, not by federal map.

Why are there so many towers in Needham?

What you see: The tall towers on the Needham/Newton line near Route 128 that most Boston TV antennas point toward.

Needham is Boston's TV antenna farm. WBZ-TV's Cedar Street tower went up in 1957, about 1,200 ft (366 m) tall at first, replacing a tower destroyed in 1954; by 2009 it was described as a 1,295 ft (395 m) guyed tower, with WCVB a tenant since 1972.³⁰³ Nearby stand the Cabot Street candelabra and the "FM-128" community tower, the former WHDH-TV channel 5 tower, where WCRB 102.5 moved.³⁰³ American Tower is landlord at both the Cedar Street and Cabot Street sites.³⁰²⁵⁷

The farm works as a set. After the TV repack, WGBH and WFXT made Cabot Street their main site, and the candelabra became the full-power backup for WBZ-TV, WCVB, WSBK and WGBX.⁵⁷ That is the redundancy argument for antenna farms in practice: if one antenna needs work, a station moves to its neighbor and stays on the air.

How do I tell a guyed tower from a candelabra?

What you see: One very tall, thin tower held up by sloping cables, and another whose top splits into several arms.

A guyed tower is a slender mast held upright by steel cables (guy wires) anchored in the ground around it. It uses little steel for its height, but the cables need a wide footprint of land. Cedar Street in Needham is described as a guyed tower.³⁰³

A candelabra tower is a self-supporting tower whose top divides into arms, each holding a separate antenna, like a candle holder. It lets several TV stations each have an antenna near the top instead of stacking them one above another. The Cabot Street tower in Needham is a stainless-steel candelabra that went up at the end of the 1960s.⁵⁷ Even on a candelabra, height still matters: WGBH-TV's antenna sits at the very top of the stack there.⁵⁷

FCC license records also label each structure with a type code. In the FCC's instructions for Form 601 Schedule M (2008, revised 2010), TOWER means "a free standing or guyed structure used for communications purposes," MAST means a self-support structure, POLE means a pole used only to mount an antenna, and BANT means a building with an antenna on top.³²⁹ Towers in an array are coded by count and position, so 3TA2 is the second tower in a three-tower array.³²⁹

Why do AM stations use several tall towers?

What you see: Two or more plain towers standing in a line in a marsh or field, with no visible antennas on them.

On AM, the tower is the antenna. The whole steel structure radiates, which is why AM towers are so tall and look bare. Its height is tied to the wavelength: WBZ's two towers in Hull are each a half wavelength tall at the station's old frequency of 990 kHz and stand a quarter wavelength apart.³⁰⁵

Several towers fed together form a directional array. The signals add up in some directions and cancel in others. In Hull, the two towers make a heart-shaped ("cardioid") pattern that cuts the signal over the Atlantic and sends it toward metro Boston, at 50 kW day and night.³⁰⁵ So a row of identical AM towers usually means a station is shaping its signal, often to protect another station on the same frequency or to avoid wasting power over water or empty land.

What are the flat panels on a rooftop mast?

What you see: A mast on top of a skyscraper with rectangular panels stacked up its sides.

On a downtown FM mast, those panels are often broadcast antennas, with each stacked unit called a bay. As of 2004, the Prudential Tower's mast held a six-bay panel "master" antenna that carried four FM stations at once (100.7, 104.1, 106.7 and 107.9), plus a separate ERI panel antenna for three more (92.9, 96.9 and 105.7).³⁰⁴ A master antenna is one antenna shared by several stations, whose transmitters are combined before the signal goes up the mast.

Not every panel on a roof is FM. Cell carriers also use panel antennas, and the two can look similar from the street. How carriers build their networks is in cellular, and the sister guide Street Code covers small cell equipment on poles.

The FCC describes the "sector" antennas cell carriers commonly use in urban and suburban areas as rectangular panels "about 1 by 4 feet in size," usually arranged in three groups of three; some base stations instead use pole-like antennas 10 to 15 ft (3 to 4.6 m) long.²⁸³ Point-to-point microwave antennas are "usually circular dish or rectangular in shape" and send a directed beam to a receiving antenna.²⁸³ The FCC says cell towers are typically 50 to 200 ft (15 to 61 m) high.³³⁰

What carries cell signals into a tunnel?

What you see: A thick cable running along a subway tunnel wall, and small antennas on station ceilings.

Inside tunnels and big buildings, carriers use a distributed antenna system (DAS): many small antennas fed from a central point. In the MBTA's system, carrier signals are converted to light, carried over fiber, then turned back into radio.⁷³ The 2012 build-out used 71 remote amplifier sites, 354 antennas and 41,585 ft (12.7 km) of radiating cable, also called leaky feeder: coaxial cable with slots that let signal leak out along its whole length.³⁰⁷ A company that owns a network like this and lets several carriers share it is called a neutral host.

Which towers are in the FCC's public database?

What you see: A tall tower on a hill or near an airport, and a question about whether it is on record.

The FCC's Antenna Structure Registration (ASR) applies to structures more than "200 feet above ground level" (61 m), and to shorter ones that may interfere with the flight path of a nearby airport.³³¹ Exemptions are listed in 47 CFR 17.7(e).³³¹ Each registered structure gets an Antenna Structure Registration Number (ASRN).

The flip side: a tower under 200 ft that is not near an airport generally does not need to be registered, so many shorter cell towers and most rooftop and pole-mounted antennas will not appear in ASR. For those, the license records in ULS are the better route.

The FCC no longer mails hard-copy registrations; the public searches online.³³²

How do I look up a tower near me?

What you see: A tower you pass every day, and a question about who owns it.

1. Open ASR Registration Search. The FCC's ASR page offers a public Registration Search and an Application Search.³³²
2. Search by location. You can search by coordinates (NAD83) with a radius, by city and state, or by a height range. If you already know it, you can also use the registration number, the FAA study number, or the owner's name or FCC Registration Number (FRN).³³³
3. Read the record. It names the owner and gives the structure's details. The owner's Taxpayer Identification Number is never shown.³³³
4. Check for broadcast stations. The Media Bureau's "ASRN within radius" tool takes a latitude, longitude and radius, and the AM, FM and TV Query results link to registered structures within 0.5 km (about a third of a mile) of a station.³³⁴

How do I find out who holds a radio license?

What you see: A tower or rooftop antenna that is not tall enough to be registered, or a radio system you hear on a scanner.

The Universal Licensing System (ULS) is the FCC's database of wireless licenses. Its License Search offers a basic search (call sign, FRN or name), an advanced search, a geographic search by radius, market-based searches, site-based searches (including by ASR number and by frequency), buildout searches and service-specific searches.³³⁵ Results can be mapped or downloaded.³³⁵

Broadcast stations are handled separately, in the Media Bureau's AM, FM and TV Query tools, which link back to ASR records.³³⁴

A license tells you who is authorized to transmit, on which frequencies and from where. It does not tell you what anyone is saying, and listening rules are a separate matter, covered by jurisdiction in scanners.

In Boston

To find the Needham towers, run an ASR search on coordinates of about 42.310 N, 71.237 W with a small radius. WBZ-TV's Cedar Street site is at about 42°18′37″N 71°14′12″W.³⁰² The records give official heights, which matter here because the trade and reference sources do not always agree: for WBZ's AM towers in Hull, Wikipedia gives 520 ft (158 m) and Fybush gives 500 ft (152 m).³⁰⁶³⁰⁵ What each of these sites broadcasts is in Boston.

Elsewhere in the world

The antenna farm pattern is common, though the standard examples cited here are all American: Mount Wilson and Sutro Tower in California, alongside Needham.³²⁵ Outside the US, national regulators such as Canada's ISED, the UK's Ofcom and Australia's ACMA handle radio licensing, and each runs a public lookup.

Canada's ISED offers Spectrum Management System searches for radio licenses by license number, call sign, geographic area and frequency, plus a broadcasting certificate search.³³⁶ In the UK, Ofcom's Wireless Telegraphy Act Register shows who is licensed in which frequencies and areas, with power and antenna details by site, but it does not show mobile base station or TV transmitter locations.³³⁷ Australia's Register of Radiocommunications Licences lists current licenses, licensees and sites, and can be searched by site, location, frequency and licensee.³³⁸ In Japan, the Ministry of Internal Affairs and Communications (MIC) links a Radio Station Information Search from its regional bureau pages, hosted on its Radio Use Portal.³³⁹

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19Test yourself

The short version: Fifteen questions drawn from this guide, from easy to surprising. Each answer explains itself and links to the section that covers it.

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20The glossary

The short version: Every term of art in this guide in plain words, searchable and sorted by band and topic. If a word on a spec sheet, a license or a scanner forum stopped you, it should be here.

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21Where to go next

The short version: The best places to keep going, none of them ours: the regulators and their public databases, the standards bodies, the hobby clubs and the live maps.

22Open questions and known gaps

The short version: What we looked for and could not pin down well enough to print. If you can source any of it, the corrections desk below wants to hear from you.

23Corrections and updates

The short version: This guide will have mistakes in it, and the people who know best are the ones who run these systems every day. If something here is wrong, out of date or missing credit, tell us and we will fix it and log the change below with the date.