Many overseas broadcast stations welcome contact with listeners.
This helps them to establish their listener profite and assess their
coverage. The station will usually repay the listener with give-away
goodies and a QSL card (from the international Q code meaning
“acknowledge receipt”). Many people make a hobby of collecting QSL
cards from
all
over the world.
Some transmissions will not contain speech or music, they
will
just
consist of warbling tones. These contain some form of data. It may be
a weather chart broadcast to shipping, or a fax or telex to a news
agency. Without a special decoder it cannot be read.
Another form of transmission found on short-wave is single side band
(SSB).
To understand what this means it helps to see how it developed. In a
conventional signal a carrier is transmitted. The amplitude (height
of
the wave) of the carrier is modulated with speech, hence the name
“amplitude modulation” (AM).
If
the signal is studied carefully then the
result of the modulation produces a carrier, an upper sideband of
frequencies and a lower sideband of frequencies.
All
this takes up space on the radio spectrum. Now, since the upper
and lower sidebands are mirror images of each other it’s not
necessary to transmit both, so one is filtered out in the transmitter. As
no information is provided by the carrier that also can be filtered out
leaving only one of the sidebands. This takes up less space in the
spectrum and, because only wanted information is transmitted, makes
better use
of the power available.
This is single sideband or SSB. The downside of this is, firstly, the
quality of reproduction is not usually as good as AM. For this reason it
is only used for communications and not for broadcast. Secondly, it is
more difficult to recover the original speech than in AM.
When recovering an SSB signal, the listener must know which
sideband is being transmitted. Fortunately there is a convention.
Frequencies below 10 MHz transmit the lower sideband (LSB), and
those above 10 MHz use the upper sideband (USB). (There are
exceptions to this convention, e.g. the R.A.F. VOLMET on 4717 kHz)
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