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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