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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