Ham Radio AM Operating: Calling Frequencies, Bandwidth and Audio

On most days the phone segments of the HF bands are filled with single sideband, and that is for good reason. But if you tune around 3.885 MHz on a winter evening or 14.286 MHz on a summer weekend, you may hear something different: voices with a full, warm sound, a steady carrier under them and a very natural tone that SSB cannot quite match. That is amplitude modulation, the oldest voice mode on the air, and a small, enthusiastic community keeps it alive. This guide to ham radio AM operating explains how the mode works, why it behaves differently from SSB, where the traditional AM calling frequencies are, how much power and bandwidth to expect and how to get good audio with modern or vintage equipment.

How AM Works

Amplitude modulation varies the strength of a radio-frequency carrier in step with the audio. The amplitude modulation article describes the principle in detail. For an operator, the result is a signal with three parts: a carrier at the centre frequency, an upper sideband and a lower sideband. The carrier carries no information of its own, but it makes reception simple, because a receiver can recover the audio with a basic detector.

The cost of that simplicity is efficiency. At full modulation, about two thirds of the total transmitted power is in the carrier, and only about one third is in the two sidebands that carry the voice. A single-sideband transmitter, described in the single-sideband modulation article, removes the carrier and one sideband and puts all the power into the other, which is why SSB reaches farther on the same power and uses less spectrum.

Bandwidth is the other difference. An AM signal occupies roughly twice the highest audio frequency, so a voice limited to 3 kHz of audio takes about 6 kHz of spectrum, and a wide hi-fi transmitter takes more. An SSB signal fits in about half of that. Both are legal in the phone segments, but an AM signal needs room.

AM Compared With SSB

The table sets the two modes side by side. It does not mean that one is better; they are tools for different situations.

Because SSB has a similar effective power with far less bandwidth, it is the standard for DX. AM is the choice for the operator who values sound quality, who enjoys restoring classic equipment or who wants to chat with a group of friends on a regular frequency. For readers who use SSB daily, our guide to upper and lower sideband explains which sideband goes where and why, and that knowledge carries over to understanding how AM sidebands relate to the carrier.

AM Calling Frequencies

There is no formal AM sub-band in most of the HF phone segments, but there are well-known gathering places. The ARRL band plan, which applies in the United States, lists AM calling frequencies near 3.885 MHz on 80 metres, 7.290 MHz on 40 metres and 14.286 MHz on 20 metres, and an AM segment on 10 metres from 29.000 to 29.200 MHz. You can see the full list on the ARRL band plan page.

These are calling frequencies by tradition, not exclusive channels. Operators call there, find a friend and then move up or down the band to continue the conversation. A voluntary band plan only works when everybody respects it, and a calling frequency is not a place for a long chat. Move away once you have made contact.

Band plans are specific to regions and countries. If you live outside the United States, check the plan of your national society and the IARU region you belong to. Our article on the IARU band plans for Regions 1, 2 and 3 explains how the world is divided, and it is a reminder that a frequency used for AM in one region may be used for something else in another. Also check your licence conditions to confirm which modes and bandwidths you may use on each band.

Where AM Is Most Active

Activity is highest on the lower bands in the evening and at night, when local and regional propagation is strong and noise levels are low enough to appreciate the audio. The 80 and 40 metre bands are popular, and the content of our guides to the 80 metre band and the 40 metre band explains why they suit regional conversation. On 20 and 10 metres, AM appears on weekends and in nets, and the 10 metre segment gets busier when the band is open.

Power, Carrier and PEP

A common surprise for newcomers is how little power an AM signal seems to produce on a radio that is rated at 100 W for SSB. The reason is the carrier. On AM, the transmitter produces a steady carrier all the time, and that heats the final amplifier far more than speech peaks do in SSB. To protect the amplifier, most modern transceivers limit the AM carrier power to a fraction of the SSB rating, often a quarter or so. A radio rated at 100 W PEP will often run about 25 W of carrier on AM.

That figure makes sense when you look at the peaks. With full modulation, the peak envelope power of an AM signal is four times its carrier power. A 25 W carrier therefore peaks at 100 W, which is the same PEP as the radio’s SSB rating. Do not be disappointed by the number on the dial; it is not a weak transmitter, it is a transmitter that is respecting the physics of the mode. The licence power limit for your country is usually expressed as PEP, so check that you remain within it.

The meter in the radio will show the carrier. Tune the transmitter into a dummy load first, as our guide to the dummy load describes, and adjust it for the correct output. Do not turn the drive up to chase a higher reading, because a distorted AM signal is wide and unpleasant on the band.

Audio Quality and Modulation

The appeal of AM is the sound, so spend time on the audio. The best-sounding AM stations have a clean microphone, a flat response and a modest amount of processing. Overmodulation is the principal sin. If the modulation exceeds 100 percent on negative peaks, the carrier is briefly cut off, and the result is distortion and splatter, which spreads the signal to adjacent frequencies. Keep the peaks below the limit and use a modulation monitor or a second receiver to check. Most modern radios have a microphone gain control and a compressor, and the setting that works for SSB does not necessarily work for AM. Our article on SSB audio settings explains the logic of gain and compression, and the same cautions apply to AM, with the difference that you should be more conservative.

• Use a good microphone and keep it a consistent distance from your mouth.
• Adjust the microphone gain so that speech peaks reach about the maximum, not beyond.
• Use only light compression, if any. Heavy compression raises the noise and makes the signal tiring.
• Set the transmit audio bandwidth sensibly. Many radios let you adjust the high and low cut. Very wide audio sounds good only when the band is quiet, and it takes up room that others need.
• Ask for a signal report from a friend, and listen to a recording if you can.

Vintage and Home-Built Transmitters

A large part of AM activity involves older equipment. Valve transmitters from the mid twentieth century, with high-level plate modulation, are prized for their sound and for the pleasure of keeping them running. Operators also build their own modulators and transmitters from surplus parts. These stations deserve respect, and so do the safety rules. High-voltage supplies in valve equipment are dangerous, and anyone who restores a vintage transmitter should follow safe practice, work with someone experienced if possible and never work on a live chassis.

Receiving AM Well

An AM receiver is simple, but a good result needs a few steps. Select AM mode, choose a filter wide enough for the audio, often around 6 kHz, and avoid tuning too far from the carrier. If two carriers are close together, you will hear a whistle at the difference frequency, called a heterodyne. A narrower filter, a slight tuning offset or a notch filter can reduce it. The filter choices are explained in our article on the receive filter bandwidth.

Many modern radios offer synchronous AM detection, which locks onto the carrier and recovers the audio with less distortion and a reduced susceptibility to fading. It can also let you choose one sideband when an adjacent station interferes with the other. If your radio has it, try it on a fading signal and compare.

The AGC setting matters on AM, because the carrier level is the control signal. A medium or slow setting usually gives pleasant audio. For help with that setting, see the notes on AGC in the receiver guides on this site.

Getting Started With a Modern Transceiver

You probably already own everything you need. Almost every modern HF transceiver includes AM mode, and most let you change the receive filter and the microphone gain separately for each mode. A simple first session looks like this:

1. Choose a quiet evening and one of the lower bands. Tune to the calling frequency for that band and listen for several minutes to learn who is active.
2. Switch the radio to AM, set the filter to about 6 kHz and leave the noise reduction off at first, so that you hear the natural sound.
3. Check the transmit settings in the menu. Make sure the carrier power is set to a sensible value and that the speech processor is off or low.
4. Call CQ briefly, or answer a station that has finished its transmission. Ask for an honest audio report and note what the other operator says about the sound.
5. Adjust one setting at a time and ask for another report. Write down the settings that gave the best result for future sessions.

Do not expect to be heard across the world with 25 W of carrier. Expect pleasant regional contacts, and enjoy the quality of the audio. If a station answers you with a very different tone than you expected, remember that propagation, the receiver filter and the other operator’s settings are all part of the result.

Etiquette on the AM Frequencies

1. Listen before you transmit. Ask whether the frequency is in use, and wait for an answer.
2. Keep calls short on a calling frequency and move away to continue.
3. Remember your bandwidth. Leave room for the neighbours and avoid putting your carrier within a few kilohertz of another station if you can help it.
4. Be tolerant of vintage equipment. A station that drifts a little or has a rough audio quality may be a lovingly maintained classic.
5. Identify with your callsign as your licence requires, and keep the conversation friendly. The AM community values good manners.
6. Do not use AM in a segment where your licence or the band plan does not allow it.

The Bottom Line

AM is not as efficient as SSB, and it will not win a DX contest, but it has an audience because of its sound, its simplicity and its connection to radio history. The mode puts most of its power into the carrier, takes about twice the bandwidth of SSB and is best suited to regional contacts on the lower bands. Find the traditional calling frequencies in your band plan, check your licence conditions, set the carrier power with a dummy load and keep the audio clean and below 100 percent modulation. Whether you use a modern transceiver or a restored valve transmitter, listen first, be considerate and enjoy a mode that sounds like nothing else on the bands.

Frequently Asked Questions

What are the AM calling frequencies?

The ARRL band plan lists AM calling frequencies near 3.885 MHz, 7.290 MHz and 14.286 MHz, and an AM segment on 10 metres from 29.000 to 29.200 MHz. Band plans differ between regions, so check yours.

Why is AM less efficient than SSB?

Because at full modulation about two thirds of the power is in the carrier, which carries no voice information. SSB removes the carrier and one sideband and puts the power into a single sideband.

How much bandwidth does an AM signal use?

About twice the highest audio frequency, so roughly 6 kHz for 3 kHz audio. That is about double the space taken by a typical SSB signal.

Why is my transceiver’s power lower on AM?

The steady carrier heats the final amplifier, so radios limit AM carrier power to a fraction of the SSB rating, often about a quarter. With full modulation the peak power then equals the SSB PEP rating.

Can I use AM on any HF band?

Only where your licence and national rules permit the mode and the bandwidth, and preferably in the part of the band where AM is customary. Check your regulations and your band plan before transmitting.

How do I avoid distorted AM audio?

Keep the microphone gain low enough that modulation peaks stay below 100 percent, use light processing and monitor your signal with a second receiver or a friend’s report. Overmodulation makes the signal wide and harsh.