AM
| AM (amplitude modulation) | |
|---|---|
| Mode type | Analog voice; full carrier with two sidebands |
| Emission designator | A3E |
| Bandwidth | About 6 kHz for normal voice audio |
| Gave way to | SSB, from the 1950s and 1960s |
| On the air | |
| Well-known US frequencies | 3.885 MHz (75 m), 7.290 MHz (40 m), 29.000–29.200 MHz (10 m) |
| Typical gear | Restored tube transmitters, homebrew rigs, modern transceivers with an AM mode |
| Power | PEP is four times the carrier power at full modulation |
| Outside ham radio | AM broadcasting, aircraft radio, CB |
AM, or amplitude modulation, is the oldest voice mode in radio and the one used by AM broadcast stations. The transmitter sends a steady carrier and varies its strength, or amplitude, in step with the sound of the voice. For decades it was the standard voice mode on the amateur HF bands, until single sideband replaced it in the 1950s and 1960s. Today AM is a specialty, kept alive by hams who enjoy restoring vintage transmitters and who value its natural, full-bodied audio, and it can be heard every evening on the lower HF bands.
How it works
When a carrier is amplitude modulated, the result is three parts: the carrier itself, on the frequency the radio is tuned to, and two sidebands, one above and one below. Each sideband carries a complete copy of the audio, one the mirror image of the other. The signal is therefore twice as wide as the highest audio frequency sent, about 6 kHz for ordinary voice audio of 3 kHz, roughly twice the width of an SSB signal.
AM is simple to receive. Any receiver with an AM mode recovers the audio by following the signal's strength, and there is no need to tune it precisely, as there is with SSB. It is less efficient than SSB, however. The carrier carries no information, yet even at full modulation it takes about two thirds of the transmitted power. An AM signal's peak envelope power (PEP) at full modulation is four times its carrier power, so the US limit of 1,500 watts PEP corresponds to a carrier of about 375 watts, and a typical 100-watt transceiver puts out a carrier of about 25 watts on AM.
Vintage and homebrew gear
Much of the AM community revolves around boatanchors, the heavy tube transmitters and receivers of the 1930s to the 1960s from makers such as Collins, Hallicrafters, E.F. Johnson and Heathkit, many of them restored to working order. Classic AM transmitters used plate modulation, in which a large audio amplifier varies the supply voltage to the final RF amplifier tube. Other operators build their own transmitters, including modern solid-state designs that are far more efficient than tubes, and most current HF transceivers and software-defined radios include an AM mode.
AM operators often take pride in audio quality, using good microphones and audio processing to produce what is sometimes called broadcast-quality sound. Wider audio means a wider signal, so the best practice is to keep the signal to a reasonable width on crowded bands.
Where to find AM
In the US, AM is permitted in the voice (phone) segments of the HF bands, except on 60 meters, where voice is single sideband within a 2.8 kHz limit. HF AM operation needs a General license or higher; on 6 meters and above, Technicians may use it too. By custom, AM gathers on a handful of frequencies, often called AM windows:
- 3.885 MHz and nearby on 75 meters, the best known, busiest in the evening.
- 7.290 MHz on 40 meters.
- 29.000–29.200 MHz on 10 meters.
AM is also heard on 160 meters, 20 meters and 6 meters; check the current band plan and listen to find the local activity.
AM etiquette
AM conversations are often relaxed roundtables, several stations taking turns, and many have a regular schedule. Because an AM signal is wider than SSB, both sides share the job of keeping the peace:
- AM operators should avoid overmodulation, which causes splatter well beyond the normal signal, and keep the signal no wider than needed when the band is busy.
- SSB operators should recognize the AM windows and avoid setting up right next to an AM group, just as they would avoid a net.
- Everyone should tune up on a clear frequency at reduced power, not on top of a conversation.
To join an AM roundtable, listen first, then call in during a break, using the radio's AM mode rather than SSB. AM operators normally zero-beat before transmitting, matching their carrier exactly to the frequency of the group, so that no whistle is heard between carriers.
AM outside ham radio
AM is still used well beyond amateur radio. Besides AM broadcasting, the VHF aircraft band, around 118 to 137 MHz, uses AM voice, partly because two stations transmitting at once can both be heard instead of the stronger blocking out the weaker, as happens on FM. The Citizens Band near 27 MHz also uses AM. Neither is an amateur band, but both are within reach of many scanners and general-coverage receivers.