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
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" [email protected] To: [email protected] 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 [email protected]. 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
----- Original Message ----- From: "Grant Hodgson" [email protected] To: [email protected] Sent: Friday, January 05, 2007 11:35 AM Subject: [amsat-bb] Linear link budgets
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.
regards
Grant G8UBN
Hi Grant, G8UBN
I aegree. As you remember AO40 had a U band uplink in 70 cm and another L band uplink in 23 cm and both where operating at the same time in certain part of the orbit.
Using my spectrum analyser connected to 144 MHz IF of my 2400 MHz downconverter it was possible for me to observe what you stated using the following procedure:
Uplink a CW carrier in 70 cm and get a reference level on spectrum analyzer at 2 dB/division for better definition and than switch off 70 cm
Uplink a CW carrier in 23 cm and adjust the output power to get the same reference level on the same frequency.
Uplink again the carrier in 70 cm while carefully adjusting the frequency and possibly the phase until the reference level on spectrum analyzer rised exactly by 6 dB or 4 time in power because the voltages of each of the 2 CW carriers 70 cm and 23 cm where added togheter in the transponder IF giving a signal with double the voltage of each signal wich is 4 time in power or a 6 dB increase as you stated.
Now if the transponder was linear to get a 6 dB increase uplinking in 70 cm alone or in 23 cm alone it would require a 4 time increase in uplink power or in 70 cm or in 23 cm from the ground station.
On the contrary uplinking at the same time in U band and L band on the same frequency less than 4 time increase in uplink power (summing U band and L band power ) was necessary to get a 6 dB increase from the mode S transponder.
Switching from CW to SSB and carefully adjusting the frequency on 70 cm or 23 cm for the minimum distortion or warbling it was possible to get a 6 dB increase in downlink with less than 4 time increase of EIRP from the ground because probably one of the two receivers on transponder was more sensitive of the other one or because the satellite antenna gain on L band was greater than the antenna gain on 70 cm
Best 73" de
i8CVS Domenico
participants (3)
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Grant Hodgson -
i8cvs -
OZ1MY