To carry speech or data you vary something about the radio wave. With AM the strength of the wave varies, with FM its frequency. That makes FM much less sensitive to noise and interference.
In practiceAviation radio (118–137 MHz) deliberately uses AM: if two stations transmit at the same time, you hear both — a squeal that warns the controller that something has been missed. FM, by contrast, lets only the strongest station through. Tactical combat net radios use FM or digital modes, because clearly intelligible speech in a noisy environment matters more there.
A bare carrier wave contains no information. To transmit speech, data or images you have to vary a characteristic of the carrier in step with the information: modulation. The two classic methods:
Amplitude modulation (AM)
With AM the frequency of the carrier stays fixed and the amplitude is varied with the information signal. AM is easy to generate and receive, but sensitive to interference: noise and interference change precisely the amplitude, and you hear that immediately. AM is used among other things in the aviation band (118–137 MHz) and on shortwave.
Frequency modulation (FM)
With FM the amplitude stays fixed and the frequency of the carrier is varied around its centre value. Because the information is carried in the frequency rather than the amplitude, FM is much less sensitive to amplitude noise: better sound quality, but it uses more bandwidth. FM is used for broadcasting and many military VHF links.
For the EW specialist, modulation matters for two reasons:
- Signal recognition: the modulation type is a fingerprint. From the modulation (AM, FM, pulses, digital variants) you can tell what kind of transmitter you are receiving, and often whose it is.
- Radar technology: radar is essentially modulation too — pulse radar modulates the amplitude (on/off), FMCW radar modulates the frequency (Variation on the CW radar). Understand AM and FM and you already understand half of the radar types.