Guide Radio 101 · 2 min
AM and FM, in one picture
Two ways of putting a voice onto a radio wave, and why one crosses oceans at night while the other sounds better across town.
Draft. Not yet checked by an editor.
A radio station sends out a steady wave at one frequency, called the carrier. On its own the carrier says nothing. To carry a voice or a song, the station changes the carrier in step with the sound. That change is called modulation, and the two oldest ways of doing it give AM and FM their names.
Amplitude modulation (AM)
In AM the strength of the carrier rises and falls with the sound. Louder sound, bigger swings. AM broadcasting sits mostly on medium wave (roughly 530 to 1,700 kHz in the Americas, 531 to 1,602 kHz elsewhere) and on shortwave.
AM is simple to receive, which is why it came first. Its weakness is noise: lightning, motors and power lines also make the signal's strength jump, and an AM receiver hears all of it.
Frequency modulation (FM)
In FM the strength stays the same and the frequency wobbles slightly up and down with the sound. Noise mostly changes strength, not frequency, so an FM receiver can ignore most of it. That is why FM sounds cleaner, and why it carries stereo and music well. FM broadcasting uses roughly 87.5 to 108 MHz.
Why AM reaches further at night
Medium-wave signals bounce off a layer of the upper atmosphere, the ionosphere, after dark, and can be heard hundreds of kilometres away. FM's much higher frequencies pass through the ionosphere instead, so an FM station is heard about as far as its antenna can see, a few dozen kilometres.
Key terms
- Carrier: the steady wave a station transmits.
- Modulation: changing the carrier to carry sound.
- kHz and MHz: thousands and millions of cycles a second.
Check yourself
01In AM, what part of the carrier changes with the sound?
Its strength (amplitude).
02Why does FM usually sound cleaner than AM?
Most noise changes a signal's strength, and an FM receiver listens for changes in frequency instead.
03Why can you hear distant AM stations at night?
After dark the ionosphere reflects medium-wave signals back to Earth far from the transmitter.