CRAC Class C (2025) — Questions 361–370

VHF/UHF local operation (repeaters, simplex) overwhelmingly uses FM (frequency modulation) for its noise immunity and simplicity. SSB is used on VHF/UHF mostly for weak-signal work, PSK is a digital HF mode, and AM is rarely used on these bands.

For weak-signal / long-distance VHF-UHF voice (e.g., troposcatter, EME, DX), SSB is preferred because of its narrower bandwidth and better weak-signal performance. FM needs stronger signals; AM/PM are not the standard.

HF voice is dominated by SSB (single sideband) because of its narrow bandwidth and efficiency; FM is impractical on HF (it needs wide bandwidth and line-of-sight-like conditions), and AM is obsolete for phone.

SSB's main advantage over FM is a much narrower transmit bandwidth (roughly 2.4–2.7 kHz vs FM's ~12.5 kHz), allowing more efficient spectrum use. B, C, D are false: co-channel stations still interfere, SSB is not immune to static, and SSB is sensitive to carrier-frequency error.

Per the source key, the main disadvantage of FM relative to SSB given here is that FM cannot simultaneously monitor (copy) multiple signals on the same channel — its capture effect means only the stronger signal is heard. (Note: D is also a real characteristic of FM, but the official answer key for this item marks A.)

On VHF/UHF, SSB is conventionally upper sideband (上边带, USB). Lower sideband is used on HF below 10 MHz; suppressed/vestigial sideband are not the standard choices here.

On HF at 14 MHz and above (20 m, 15 m, 10 m, etc.), voice uses upper sideband (USB). Below 10 MHz, lower sideband is used. 14 MHz is the threshold where the convention switches to USB.

On 40 m and lower (7 MHz and below, e.g., 80 m, 160 m), voice uses lower sideband (LSB). This is the standard amateur convention matching the 14 MHz/10 MHz threshold rule.

The sideband split at 10.1 MHz is purely a Convention agreed by amateurs worldwide (a band plan habit), not a propagation effect. B and C are false (propagation is independent of sideband choice), and D is technobabble.

An SSB signal is about 2.4–2.7 kHz wide, so to avoid overlapping nearby stations you should keep at least 2–3 kHz separation. B/C are CW/digital spacings, and D (~6 kHz) is wider than necessary.