FM
| FM (frequency modulation) | |
|---|---|
| Mode type | Voice; the frequency of the carrier varies with the audio |
| Bandwidth | About 12–16 kHz (amateur FM voice) |
| Where used | 2 m, 70 cm, other VHF and UHF bands; a small segment of 10 m |
| Strengths | Clear, noise-free audio; simple to tune; works with cheap radios |
| Weaknesses | Needs a fairly strong signal; wide bandwidth |
FM, frequency modulation, is the voice mode of local amateur radio. Every handheld, every mobile radio and almost every repeater uses it. It is the same mode as FM broadcasting and public-safety radio, though hams use a narrower version, so the audio is clean and free of the static that plagues AM and the tuning fuss of SSB.
How it works
An FM transmitter keeps its power constant and shifts the carrier frequency up and down in step with the voice: louder audio means a bigger shift, called deviation. Amateur FM voice normally uses a deviation of about 5 kHz, far less than broadcast FM; some repeaters and radios use a narrower setting of about 2.5 kHz. The receiver's limiter strips off any amplitude variations, which is where most noise lives, so a good FM signal is either clear or it isn't there at all. That is the "capture effect": the stronger of two signals on the same frequency wins outright, and a weak FM signal disappears into noise rather than fading gracefully.
Because channels are fixed and radios tune in steps, FM operating is channelized. You don't tune around; you pick a memory channel.
Where you'll use it
- Repeaters on 2 meters and 70 centimeters, the way most hams talk locally.
- Simplex on the national calling frequencies and nearby channels.
- Nets, most of which are FM on local repeaters.
- Satellites. Several amateur satellites and the International Space Station carry FM repeaters that a handheld and a small Yagi can work.
- 10 meters, where a segment from 29.5 to 29.7 MHz is set aside for FM and a few repeaters.
Tones and squelch
FM radios have a squelch that mutes the speaker until a signal arrives, so you don't listen to hiss. CTCSS (PL) tones and DCS codes go a step further: your radio can be set to open only for signals carrying the right tone, and repeaters use the same tones to decide whether to key up. Setting the transmit tone correctly is the step new hams most often miss.
Digital voice
Digital voice modes such as DMR, D-STAR and System Fusion are replacing FM on some repeaters. They encode speech into data, which lets repeaters link over the internet and adds features like text and GPS position, but the audio is either perfect or chopped up, and each system needs its own radios. FM remains the common language that every ham radio speaks.