G2A06 from the General License Course book, Section 4.1 AM & SSB:
Q. Which of the following is an advantage when using single sideband as compared to other analog voice modes on the HF amateur bands?
A. Very high fidelity voice modulation
B. Less bandwidth used and higher power efficiency
C. Ease of tuning on receive and immunity to impulse noise
D. Less subject to static crashes (atmospherics)
Let’s get at this question with an examination of single sideband characteristics in comparison to other voice modes and the correct answer will pop right out at us. First we’ll review a couple of basic concepts about radio transmissions.
A radio transmission is comprised of a band of frequencies. That is, when you push-to-talk and speak into the microphone a whole range of frequencies is transmitted (bandwidth), not just that singular frequency that is depicted on your transmitter’s display.
That displayed carrier frequency is just a sort of reference point to help indicate what transmitted band of frequencies you are actually using. You’re usually transmitting a bandwidth of several thousand hertz with voice modes like AM, FM, or SSB. Your voice is comprised of a range of audio frequencies, from low tone vowel sounds like “Oh” and “Ah” to high tone, crispy consonant sounds like “Ch, P, S, or D.” As a result, your voice typically uses a band of several thousand hertz of audio frequencies when you speak, so a commensurate band of radio frequencies is required to represent your voice in light-speed RF transmissions.
Frequency Modulation: FM voice transmissions vary in how much bandwidth is used. Drive the signal hard by speaking loudly into the microphone and you will use more bandwidth than if you softly mumble. This is because the amplitude (power) of your audio signal determines how broadly the FM radio frequency deviates from the resting carrier frequency value. But a typical FM voice transmission uses from 5 kHz to 15 kHz of bandwidth (or more, in actual practice). This relatively large frequency band provides very good quality audio, or relatively “high fidelity voice modulation,” by encoding lots of high highs and low lows of voice audio information into the wide band of FM radio signals. Coupled with the fact that it is less susceptible to atmospheric noise or “static crashes” such as those caused by lightning, FM sounds really nice to our ears.
Amplitude Modulation: AM voice transmissions have a fixed bandwidth of about 6 kHz. The AM signal is comprised of two mirror image bands, one band either side of the carrier frequency displayed on the radio. The two bands of AM reinforce one another and help to provide a robust audio signal. However, the bandwidth is narrower than FM, so the fidelity of the transmitted AM signal is less than the typical FM signal. And since AM signals are subject to atmospheric static the quality of the resultant audio is reduced relative to FM.
Single Sideband Modulation: SSB is a special form of AM that uses only one of the two mirror image AM bands. Complete voice information is contained in either one of these AM sidebands, so a sufficient voice signal may be sent using half the bandwidth of AM, or about 3 kHz. Why is that important? Two primary reasons:
1. When each operator uses less bandwidth more signals will fit into the overall amateur band. In a geographic area where “everybody can hear everybody else,” SSB would allow double the number of QSOs on a limited amateur band without interference than would AM, and about five times more than FM would allow.
2. The power of your transmitted signal must be used to generate the band of frequencies for carrying your voice on the air. You have a finite amount of power with which to transmit. When you transmit with a broad bandwidth that power is spread across the production of all those many frequencies, so the power allocated to each frequency is low. But if you have a narrow bandwidth, perhaps only half as wide, the power allocated to the production of each frequency will be doubled. Narrower bandwidth uses your power more efficiently than broad bandwidth, boosting your signal strength.
Now you can answer this question easily. The advantage of SSB is its narrow bandwidth and higher power efficiency than the other voice modes.
The Other Options: The wide bandwidth of FM provides a higher fidelity, while the fidelity of SSB suffers with narrower bandwidth, so option ‘A’ is out. Being a special form of AM, SSB is still subject to atmospherics and static crashes, so ‘D’ is eliminated. Option ‘C’ is out because SSB (and AM) is more difficult to tune within a continuous band rather than the typical discrete channels assigned in most FM operations, and impulse noise like the clicks of an automobile’s alternator will often infect FM, AM, and SSB alike.
For question G2A06, “Which of the following is an advantage when using single sideband as compared to other analog voice modes on the HF amateur bands?” the correct answer is B. Less bandwidth used and higher power efficiency.
Related Questions: G2A01 through G2A07, G2B05