T8A

FM, SSB and signal bandwidth

How AM, FM and SSB differ, how wide each signal is, and which to choose for repeaters, weak signals and sidebands.

12 pool questions1 on the exam

Syllabus: Basic characteristics of FM and SSB; Bandwidth of various modulation modes: CW, SSB, FM, fast-scan TV; Choice of emission type: selection of USB vs LSB, use of SSB for weak signal work, use of FM for VHF packet and repeaters

AM changes the height. FM changes the frequency.

Modulation means putting your voice onto a steady carrier wave. AM varies the carrier's height. FM varies its frequency. PM (phase) is FM's close cousin.

SSB is a form of AM: the carrier and one sideband are removed. FM or PM is what VHF/UHF repeaters and VHF packet use.

RememberSSB belongs to the AM family. Repeaters and VHF packet use FM (or PM).

Narrowest to widest: CW, SSB, FM, TV

Bandwidth is the slice of spectrum a signal occupies. Less information, less bandwidth.

Longer bar = wider signal. Log scale: every tick is ten times the last, so TV is far wider than it looks.

Memory hook150 Hz, 3 kHz, 10–15 kHz, 6 MHz: a switch, a voice, a wide voice, a whole picture.

SSB: narrow, good for weak signals. FM: simple, one at a time.

	SSB	FM
Bandwidth	Narrower (3 kHz)	Wider (10–15 kHz)
Weak signals	Best. Less noise gets in	Needs a stronger signal
Two on one channel	Often both audible	Only one is received
Typical use	Long-distance VHF/UHF	Repeaters, local

RememberNarrow wins for distance. FM's catch: only one signal at a time.

Above 10 MHz, use the upper sideband

An SSB signal is the upper or lower half of the usual signal around the carrier. By convention hams use LSB below 10 MHz and USB above, which includes 10 m, VHF and UHF.

Choose one sideband to send. The carrier and the other side are left out.

Memory hookLow bands, lower sideband. High bands, upper sideband.

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Practise the 12 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".

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