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SSB & GEO



Re. the post about 5 SSB channels each having 2W each for a 10W
transmitter - NOT SO!

Heres why :-

Take the simple case of 2 closely spaced CW signals of equal amplitude.  If
each signal had a power level of 1W, the transmitter would be producing 4W
PEP.  This is because the peak power is generated at the instant when both
CW tones are at the peak of their sine waves.  At this instant in time, the
peak VOLTAGE for the composite signal is double the peak voltage of each
tone, so the peak power generated is 4 times that of each tone.  Therefore,
a linear transmitter that is able to generate 4W PEP woud only be able to
support 2 1W signals.  This is also true if those signals were SSB, of 1W
PEP each.

This can be extended to any number of signals quite easily.  For example,
if 4 SSB signals were present, each of 1W PEP, the transmitter would have to
have an output power rating of 16W PEP.

This is one reason why all uplink signals on AO-40 should be no more than
10dB below the beacon - at this level, the linear transponder has some
headroom to be able to cope with multiple signals without causing problems
and all is well.  As soon as multiple signals appear that are similar in
amplitude then severe problems can occur.  This could cause problems on a
simple linear transponder.

regards

Grant Hodgson
G8UBN@amsat.org

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