WEBVTT NOTE Modes and What They Cost in Bandwidth 1 00:00:00.000 --> 00:00:12.460 Two families of modulation. Every analogue voice mode does one of two things to a carrier. Amplitude modulation varies the carrier's strength with the audio. 2 00:00:12.460 --> 00:00:29.761 Single sideband is a form of amplitude modulation, the pool asks this directly. SSB is AM with the carrier and one sideband removed, which is why it fits in a third of the space and puts all the power into information. 3 00:00:29.761 --> 00:00:44.760 Frequency modulation varies the carrier's frequency with the audio. FM and its close relative PM (phase modulation) are what VHF and UHF voice repeaters use, and what VHF packet radio uses. 4 00:00:44.760 --> 00:00:57.680 The bandwidth table. There is a diagram here. Horizontal bars on a logarithmic frequency axis from 100 hertz to 10 megahertz. 5 00:00:57.680 --> 00:01:20.314 PSK31 occupies about 31 hertz, FT8 about 50 hertz, CW about 150 hertz, single sideband voice about 3 kilohertz, FM voice on VHF repeaters between 10 and 15 kilohertz, and analogue fast-scan television about 6 megahertz. 6 00:01:20.314 --> 00:01:37.265 Mode, CW; Approximate bandwidth, 150 Hz. Mode, SSB voice; Approximate bandwidth, 3 kHz. Mode, FM voice (VHF repeater); Approximate bandwidth, between 10 and 15 kHz. 7 00:01:37.265 --> 00:01:58.660 Mode, AM fast-scan TV; Approximate bandwidth, about 6 MHz. Four numbers, all asked directly. CW has the narrowest bandwidth of the signal types in the pool. The scale is worth feeling rather than memorising. 8 00:01:58.660 --> 00:02:15.920 One fast-scan television signal is about 6 MHz wide, the entire 2 metre band is 4 MHz. A single analogue TV transmission would not fit in it. Why SSB for weak signals. 9 00:02:15.920 --> 00:02:26.720 Question. Which type of voice mode is often used for long-distance (weak signal) contacts on the VHF and UHF bands? 10 00:02:27.620 --> 00:02:30.500 Answer. SSB. 11 00:02:31.100 --> 00:02:42.496 Two reasons, and they compound: Narrower bandwidth means less noise. A receiver's noise power is proportional to its bandwidth. 12 00:02:42.496 --> 00:02:58.289 An SSB receiver taking in 3 kHz admits roughly a quarter of the noise an FM receiver taking in 12 kHz does, about 6 dB of free sensitivity. FM has a threshold and SSB does not. 13 00:02:58.289 --> 00:03:14.980 An FM signal below the receiver's threshold does not get quieter, it disappears into noise. An SSB signal just gets weaker, and a human ear pulls speech out of noise remarkably far down. 14 00:03:14.980 --> 00:03:23.966 Question. What is one characteristic of single sideband (SSB) compared to FM? 15 00:03:24.866 --> 00:03:29.621 Answer. SSB signals have narrower bandwidth. 16 00:03:30.221 --> 00:03:32.222 Which sideband. 17 00:03:32.222 --> 00:03:42.982 Question. Which sideband is normally used for 10-meter HF, VHF, and UHF single-sideband communications? 18 00:03:43.882 --> 00:03:46.996 Answer. Upper sideband. 19 00:03:47.596 --> 00:04:06.756 Convention, not rule. USB above 10 MHz and on VHF/UHF; LSB on 160, 80, and 40 metres. Nobody remembers why; everybody follows it, and using the wrong one makes your voice unintelligible rather than merely different. 20 00:04:06.756 --> 00:04:24.490 Where FM wins. FM is not the poor relation, it dominates local operating for good reasons. Constant amplitude makes it immune to the impulse noise that plagues AM, and receivers are simple and cheap. 21 00:04:24.490 --> 00:04:34.738 Its distinctive property is the capture effect: when two signals arrive together, the stronger one wins completely. 22 00:04:34.738 --> 00:04:42.541 Question. Which of the following is a disadvantage of FM compared with single sideband? 23 00:04:43.441 --> 00:04:48.087 Answer. Only one signal can be received at a time. 24 00:04:48.687 --> 00:05:01.432 That is the capture effect stated as a cost. On SSB you hear two stations overlapping and can work through it; on FM the weaker one simply vanishes. 25 00:05:01.432 --> 00:05:15.727 On a busy repeater that is a feature, no mush, just whoever is strongest. In a contest or an emergency net where several stations need to be heard, it is a liability. 26 00:05:15.727 --> 00:05:33.086 Choosing, in practice. Situation, Local contact through a repeater; Mode, FM. Situation, Distant VHF contact, weak signal; Mode, SSB, horizontally polarised. 27 00:05:33.086 --> 00:05:50.776 Situation, Weakest possible signal, any distance; Mode, CW or a digital mode. Situation, Data over VHF; Mode, FM with a packet or digital modem. Check yourself. 28 00:05:50.776 --> 00:06:14.327 Rank CW, FM voice, and SSB voice by bandwidth, narrowest first. Why does SSB reach further than FM at the same power? Two stations transmit on the same FM repeater input at once. What does the repeater retransmit? 29 00:06:20.327 --> 00:06:41.793 CW (150 Hz), SSB (3 kHz), FM (10 to 15 kHz).. Narrower bandwidth admits less receiver noise, about 6 dB, and SSB degrades gracefully below the point where FM drops out entirely. The stronger one, alone. 30 00:06:41.793 --> 00:06:48.807 That is the capture effect; only one signal is received at a time.