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Re: Linear link budgets



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
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