In AM (amplitude modulation) the height of a constant-frequency carrier follows your voice. It is the oldest voice mode, the one used by broadcast stations and aircraft radios. It is wider and less efficient than SSB, but simple to generate and receive, and it carries natural, full-bodied audio.
Where the power goes
An AM signal is a carrier plus two sidebands, mirror copies of the voice above and below it. The sidebands carry all the information. The carrier carries none; it only gives a simple receiver something to compare against. At full modulation, each sideband gets a quarter of the carrier's power, so just a third of the total is voice. Drag the slider to see the split.
Spectrum heights show relative power. Even at 100% modulation, two thirds of the power is carrier.
Power in the sidebands33%
This is why SSB has largely replaced AM on crowded HF bands: it drops the carrier and one sideband, putting everything into the voice in half the space.
Why anyone still uses it
Simplicity. Old transmitters and receivers, including classic tube gear, are AM. Restoring and using them is a hobby of its own.
Audio quality. With a wide receiver filter, speech sounds natural, and many operators like the "broadcast" character.
No sideband to choose and no capture. Two overlapping stations are both heard, unlike FM.
Aviation. The civil aircraft band uses AM, so listening there is a good way to hear it.
In practice
AM activity gathers in informal "AM windows" on the HF bands, commonly near 3.885, 7.290 and 14.286 MHz. Listen for where people actually are.
Use a wide filter (about 6 kHz) on receive for full audio, or a narrower one to cut noise.
Two carriers a few hundred hertz apart produce a whistling heterodyne.
Don't overmodulate. Going past 100% distorts the envelope and spreads splatter into other channels.
The envelope is the outline of the peaks. Past full drive it flattens and the signal spreads sideways.
BandwidthNormal
Phone emissions, AM included, are permitted only in the phone portions of each band; see Band plans.