[amsat-bb] Linear link budgets

Grant Hodgson grant at ghengineering.co.uk
Fri Jan 5 02:35:25 PST 2007

Unfortunately, the calculations for dividing transponder power equally 
amongst users are not as simple as have been suggested.

For the 400mW case :- assume that 400mW is the maximum power that the 
transmitter can produce.  Then a single CW signal could in theory be 
sent at a power of 400mW, assuming no AGC, beacons etc.

However, if 2 equal level CW signals were being transmitted, then the 
Peak Envelope Power of these 2 signals would be 6dB higher than each of 
the signals individually.  This is because the voltages of each of the 2 
CW signals have to be added together, giving a peak signal with double 
the voltage of each signal, which is 4 times the power or a 6dB increase.

Therefore, to keep within the 400mW limit, each CW signal would be 
restricted to 100mW each.  The same analysis can be done for 3 equal CW 
signals, where each would be 9.5dB down; i.e. 44mW each etc.

For SSB signals, the same analysis can be performed by simply taking the 
PEP of each SSB signal; so one CW signal could co-exist with one SSB 
signal, both of which would have a downlink power of 100mW.

The above is actually a slight over-simplification, as it takes no 
account for compression etc, but serves as a useful example.

As Graham mentions, a beacon may be present at a much higher level than 
the rest of the transponder, which makes the analysis slightly more 
complex, but the principle remains :-

It's the PEAK power that limits the RF performance of linear 
transmitters, and that is valid for all linear transmitters.


Grant  G8UBN

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