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T8A

SIGNALS AND EMISSIONS

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

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T8A011 of 12

Which of the following is a form of amplitude modulation?

Why Amplitude modulation varies the strength of the RF signal with the audio. Single sideband is simply AM that has had the carrier and one of the two sidebands filtered out, leaving only the information-bearing sideband, so it is still a member of the AM family. Removing the carrier and redundant sideband cuts the bandwidth roughly in half and puts all the transmitter power into the useful signal.
Watch out Phase shift keying varies phase, not amplitude, and spread spectrum is a technique for spreading energy across a wide band; packet radio is a data protocol, usually sent over an FM transmitter, not a modulation type in itself.
SSB = AM minus carrier minus one sideband. If the name has 'sideband' in it, think amplitude.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T8A022 of 12

What type of modulation is commonly used for VHF packet radio transmissions?

Why Classic VHF packet (AX.25 at 1200 baud) sends audio tones through an ordinary FM voice transceiver, so the RF is frequency or phase modulated. FM/PM works well here because it is constant amplitude and immune to amplitude noise, and standard VHF FM rigs are cheap and plentiful. The tones are just AFSK audio fed into the mic input, so the transmitted emission type is the FM (or PM) the radio already produces.
Watch out SSB is the usual choice for weak-signal VHF work and for HF data, and some higher-speed HF packet uses it, but VHF packet gear is built around FM voice radios. PSK modes like PSK31 are separate keyboard modes, not the AX.25 packet standard on 2 meters.
Packet rides on the same 2 meter FM radio you use for repeaters: VHF packet = FM.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T8A033 of 12

Which type of voice mode is often used for long-distance (weak signal) contacts on the VHF and UHF bands?

Why SSB puts all transmitter power into a single sideband roughly 2 to 3 kHz wide, with no carrier and no redundant second sideband wasting energy. That concentrated power plus a narrow receiver bandwidth (less noise admitted) gives SSB a big weak-signal advantage, which is why VHF/UHF DX, meteor scatter and EME operators work the SSB portions near the low end of each band.
Watch out FM is the other tempting answer because it dominates VHF, but FM spreads power over 10 to 15 kHz and has a capture/quieting threshold: below a certain signal level it turns to noise rather than degrading gracefully, so it is for local repeater and packet work, not DX.
Narrow beats wide when signals are weak: SSB for DX, FM for local repeaters.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T8A044 of 12

Which type of modulation is commonly used for VHF and UHF voice repeaters?

Why Repeaters use angle modulation, FM or its close cousin PM, because the signal amplitude stays constant. That lets a receiver's limiter strip off noise and amplitude variations, and the FM capture effect makes the strongest signal dominate so you hear clean audio instead of two stations mixing. Constant-amplitude signals also allow simple, efficient transmitter amplifiers, and the wider VHF/UHF bands have room for FM's roughly 10 to 15 kHz bandwidth. PM sounds the same through an FM receiver, which is why the pool pairs them.
Watch out SSB is the right pick for weak-signal work on the VHF/UHF CW and SSB segments, but it needs precise frequency tracking and has no capture effect, so it is not used for FM voice repeaters.
Repeaters and packet = FM; weak-signal DX = SSB. Constant amplitude means quiet, captured audio.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T8A055 of 12

Which of the following signal types has the narrowest bandwidth?

Why Bandwidth grows with how much information the signal carries. CW is just an on/off keyed carrier sent at a few dozen words per minute, so it only needs roughly 100 to 300 Hz. SSB voice needs about 2 to 3 kHz to pass speech frequencies, slow-scan TV occupies a similar voice-sized channel because it is audio tones, and FM voice spreads out to 10 to 15 kHz because of the frequency deviation plus sidebands.
Watch out SSB voice is the narrowest of the voice modes, which makes it tempting, but at about 3 kHz it is still ten or more times wider than keyed CW.
Bandwidth ladder: CW (~150 Hz) < SSB/SSTV (~3 kHz) < FM (~10-15 kHz) < fast-scan TV (MHz).
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T8A066 of 12

Which sideband is normally used for 10-meter HF, VHF, and UHF single-sideband communications?

Why SSB operating convention divides the spectrum: lower sideband is customary on 160, 80 and 40 meters, while upper sideband is used on 20 meters and everything above that, including 10 meters, VHF and UHF. This is not an FCC rule, just a long-standing practice so stations can find each other without guessing. Since both sidebands carry the same information, the only thing that matters is that everyone in a band picks the same one.
Watch out Lower sideband is the right answer only for the low HF bands (160, 80 and 40 meters); the question specifically names 10 meters and up. "Suppressed sideband" and "inverted sideband" are not operating conventions at all, though an SSB transmitter does suppress the unwanted sideband and the carrier.
Below 10 MHz, go Low; above it, go Up. 10 meters, VHF and UHF are all USB.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T8A077 of 12

What is one characteristic of single sideband (SSB) compared to FM?

Why SSB transmits only one sideband and suppresses the carrier, so a voice signal occupies roughly 3 kHz. FM voice spreads the same audio over about 10 to 15 kHz because the deviation and sidebands widen the signal. That narrower bandwidth is why SSB concentrates transmitter power into a smaller slice of spectrum, making it the mode of choice for weak-signal HF and VHF work.
Watch out The claim that SSB is easier to tune is backwards: SSB needs tuning within about 50 Hz or voices sound like ducks, while FM tunes in easily anywhere in the channel. SSB is also more prone to noise and interference, since FM's limiter strips off amplitude noise.
SSB about 3 kHz, FM about 10 to 15 kHz. Narrow and fussy to tune versus wide and forgiving.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T8A088 of 12

What is the approximate bandwidth of a typical single sideband (SSB) voice signal?

Why SSB transmits only one sideband with the carrier suppressed, so its bandwidth equals the audio range being sent. Communications-quality speech is filtered to roughly 300 Hz to 3000 Hz, which puts a typical SSB voice signal at about 3 kHz wide. That is half the width of an AM signal, which carries both sidebands.
Watch out The 6 kHz figure is the bandwidth of a double-sideband AM signal, and 15 kHz is about what an FM voice signal occupies; 1 kHz is closer to a CW signal's width.
Pool bandwidths: CW 150 Hz, SSB 3 kHz, AM 6 kHz, FM 15 kHz. SSB is half of AM because one sideband is gone.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T8A099 of 12

What is the approximate bandwidth of an FM voice signal on VHF repeaters?

Why Amateur FM on VHF repeaters uses about 5 kHz peak deviation with audio out to roughly 3 kHz, and Carson's rule (bandwidth = 2 x (deviation + highest audio frequency)) gives about 16 kHz, so the usual figure quoted is 10 to 15 kHz. That is why repeater channels are spaced 15 or 20 kHz apart. FM is much wider than SSB voice (about 3 kHz) because the carrier swings back and forth in frequency.
Watch out The choice near 150 kHz describes commercial broadcast FM, which uses much larger deviation and 200 kHz channel spacing; the under 500 Hz figure is a CW signal.
Ham FM fits a 15 kHz repeater channel; broadcast FM needs 150 kHz. Same mode, ten times the room.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T8A1010 of 12

What is the approximate bandwidth of AM fast-scan TV transmissions?

Why Amateur fast-scan TV uses the same analog NTSC format as old broadcast television, so it occupies a standard 6 MHz channel: a vestigial-sideband video carrier plus a separate FM audio subcarrier about 4.5 MHz above it. That much bandwidth will not fit in any HF or VHF band segment, which is why fast-scan TV is used on 70 cm and higher. For comparison, FM voice is about 10-15 kHz and SSB voice about 3 kHz, so TV is hundreds of times wider.
Watch out The choice of about 3 MHz is roughly the video portion alone if you forget the guard space and sound subcarrier; more than 10 MHz is wider than any allocated broadcast TV channel.
ATV = a broadcast TV channel = 6 MHz, so it only fits on 70 cm and up.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T8A1111 of 12

What is the approximate bandwidth required to transmit a CW signal?

Why A CW signal is just a carrier switched on and off, so the only thing that spreads it out is the keying itself. The faster the keying and the sharper the rise and fall of each dot, the wider the sidebands, but at normal hand-sending speeds the occupied bandwidth is only on the order of 150 Hz. That narrowness is why CW receivers can use very tight filters and dig signals out of noise that would bury a voice signal.
Watch out The 2.4 kHz figure is a typical SSB voice filter width, and 15 kHz is about what an FM voice channel occupies, both far wider than what on/off keying needs.
Bandwidth ladder: CW 150 Hz, SSB ~3 kHz, FM ~15 kHz, fast-scan TV ~6 MHz. CW is the smallest number.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
T8A1212 of 12

Which of the following is a disadvantage of FM compared with single sideband?

Why FM receivers exhibit the capture effect: the strongest signal on a frequency takes over the detector and completely suppresses weaker ones, so you hear only one station even if two are transmitting. With SSB, two or more signals on nearly the same frequency all come through together and the operator can pick out the weaker one by ear. That makes FM a poor choice for crowded weak-signal work, while its wide 10-15 kHz bandwidth and capture behavior are fine for repeaters.
Watch out Voice quality is actually one of FM's strengths, since the noise-limiting detector gives clean, natural audio; SSB sounds thinner and needs careful tuning. FM is also easier to tune, not harder, because a small frequency error does not shift the pitch of the recovered audio.
FM "captures": strongest signal wins and the rest vanish. SSB lets you hear them all fighting it out.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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