Hi Grant and all, Yes you are right about the PEP - but it is not as bad as that if we take statistics into account. 10 stations are not likely to transmit at the same time. As far as I remember AMSAT-DL used the square root of the number of users as a good indication of how many users there can be on a linear transponder to calculate the power for each user. Have a nice weekend 73 OZ1MY Ib ----- Original Message ----- From: "Grant Hodgson" grant@ghengineering.co.uk To: amsat-bb@amsat.org Sent: Friday, January 05, 2007 11:35 AM Subject: [amsat-bb] Linear link budgets
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.
regards
Grant G8UBN _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb