Single-sideband (SSB) is the most popular mode of voice transmission on the
HF bands. (FM is mainly used above 50 MHz) The mode got its name because
of a key difference from the older mode, AM, which is used by AM broadcast
stations and was the original voice mode hams used. Whereas an AM transmitter
outputs two identical copies of the voice information, called sidebands,
a SSB signal only outputs one. This signal is much more efficient and saves
precious radio spectrum space.
Most voice signals on HF are SSB, so you have to choose between USB (Upper
Sideband) and LSB (Lower Sideband). The actual SSB signals extend in a narrow
band above (USB) or below (LSB) the carrier frequency displayed on the radio
(see Figure 8-2). How do you choose? By long tradition stemming from the
design of the early sideband rigs, on the HF bands above 9 MHz, voice operation
is always on USB. Below 9 MHz, you find everyone on LSB.
Because hams must keep all signals within the allocated bands, you need
to remember where your signal is actually transmitted. Most voice signals
occupy about 3 kHz of bandwidth. If the radio is set to USB, that means your
signal appears on the air from the displayed frequency up to 3 kHz higher.
Similarly on LSB, the signal appears up to 3 kHz below the displayed frequency.
When operating close to the band edges, make sure your signal stays in the
allocated band. For example, on 20-meters, the highest frequency allowed for
hams is 14.350 MHz You can tune a radio operating on USB no higher than
14.350 MHz -3 kHz = 14.347 MHz to stay legal.