T8A: FM, SSB, and Bandwidth – Ham Radio Technician License Study Guide

T8A covers the modulation modes used in amateur radio and the bandwidth each one requires. Understanding these concepts lets you choose the right mode for any operating situation and helps you read a band plan intelligently.

This group focuses on FM and SSB — how they work, where each is used, and how much spectrum each one occupies. It also covers CW bandwidth, fast-scan TV bandwidth, and the key disadvantage of FM compared to SSB.

Modulation Modes: FM and SSB

Radio transmissions carry information by varying (modulating) some property of the carrier wave. Two major modulation families appear on the Technician exam:

FM (frequency modulation) varies the frequency of the carrier in step with the audio signal. The carrier amplitude stays constant. FM is robust against amplitude noise and provides natural-sounding audio, which is why it dominates VHF and UHF voice communications and is standard for repeaters. FM and PM (phase modulation) are closely related — both are used for VHF packet radio and voice repeaters.

SSB (single sideband) is a form of amplitude modulation. Conventional AM transmits a carrier plus two sidebands (upper and lower), both carrying the same information. SSB suppresses the carrier and one sideband, transmitting only the remaining sideband. This cuts bandwidth roughly in half compared to full AM and concentrates all transmit power into the useful part of the signal. SSB is efficient for long-distance and weak-signal work.

When to Use Each Mode

Mode selection depends on what you are doing and which band you are on:

• FM or PM — VHF and UHF voice repeaters, VHF packet radio. FM is the default for local and regional communications through repeaters.
• SSB — long-distance (weak-signal) contacts on VHF and UHF, HF voice communication. When signals are marginal, SSB's narrow bandwidth and full-power efficiency make it the better choice over FM.
• Upper sideband (USB) — the convention for 10-meter HF, VHF, and UHF SSB operation. (Lower sideband is used on frequencies below 10 meters.)

Bandwidth by Mode

Bandwidth is the range of frequencies a signal occupies. Narrower signals use less spectrum and can coexist with more other signals in a given band. Here are the approximate bandwidths for modes covered in T8A:

CW is the narrowest of all common modes — it requires only about 150 Hz. SSB is narrower than FM, which is a significant advantage for long-distance work. AM fast-scan TV is the widest, occupying about 6 MHz of spectrum.

FM Disadvantage Compared to SSB

FM has one notable limitation: only one signal can be received at a time. This is called the FM capture effect — when two signals are present on the same frequency, the stronger one captures the receiver and suppresses the weaker one entirely. SSB receivers can often pick out multiple signals on nearby frequencies simultaneously. This capture effect makes FM impractical where multiple stations might transmit simultaneously, and is one reason SSB dominates weak-signal and contest operation on VHF and UHF.

T8A Practice Questions