FM — Frequency Modulation for VHF/UHF - Transmitters & Receivers

FM — Frequency Modulation for VHF/UHF

FM vs SSB — Different Approaches

While SSB varies the amplitude of the signal to carry voice, FM varies the frequency. The amplitude stays constant.

FM is used for most VHF/UHF communication (2m and 70cm repeaters). It's simpler to build, more noise-resistant, and sounds better than SSB for local communications.

Key FM Concepts

Deviation (Δf): How far the frequency swings from the carrier. For amateur narrow-band FM: ±5 kHz
Modulation index (β): Deviation divided by the audio frequency: \( \beta = \Delta f / f_m \)
Bandwidth (Carson's rule):

\( BW \approx 2(\Delta f + f_m) \)

Example: With ±5 kHz deviation and 3 kHz maximum audio: BW = 2(5 + 3) = 16 kHz. That's why FM channels on 2m are spaced 12.5 or 25 kHz apart.

The FM Capture Effect

One of FM's biggest advantages: when two stations transmit on the same frequency, the stronger signal wins completely. The weaker signal is suppressed, not mixed in.

Why this matters for repeaters: The capture effect is why FM works so well for repeaters. If two stations accidentally transmit at the same time, you hear the stronger one clearly — not a garbled mix of both. With SSB, you'd hear both simultaneously.

Pre-emphasis and De-emphasis

FM noise gets worse at higher audio frequencies. To combat this:

Transmitter (pre-emphasis): Boosts high audio frequencies before transmission
Receiver (de-emphasis): Cuts high audio frequencies after demodulation

The boost and cut cancel out for the wanted signal, but the noise (which wasn't boosted) gets cut. Result: better signal-to-noise ratio.