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AO-40 Transponder Noise and effects on system sensitivity



In response to a number of recent postings on the BB regarding beacon 
strength, transponder noise etc, I am working on a spreadsheet to calculate 
the effect of transponder noise on system sensitivity.
As the quality of a receiving system increases, either due to a larger 
antenna or a lower system noise figure, the contribution of the transponder 
noise to the overall receiver noise increases, to the point where the 
transponder noise becomes dominant. When this happens, there is little 
point in improving the receiving system as the overall signal to noise 
ratio is more or less set by the satellite transponder, not your receiver. 
The spreadsheet, when finished, should enable this point to be determined 
for individual systems. It will hopefully enable stations to determine how 
much more improvement they could make in their receiving systems during the 
forthcoming off-pointing season.
During the next few passes, I am hoping to take some reasonably accurate 
measurements near apogee of the transponder noise relative to cold sky, and 
the level of the beacon. It would be very helpful to this study if others 
could do the same, to add more data points to refine the model.
The data needed is
Date/time of measurement
MA
Squint (if known)
RX noise level with antenna pointed towards clear sky
RX noise level with antenna pointed at the satellite with transponder ON, 
on a clear frequency as near as possible to the beacon (say 10-20kHz below 
the beacon)
Level of the beacon (either dB relative to clear sky or dB relative to 
transponder noise)
System noise figure or type of downconverter
Antenna gain or type of antenna (eg dish diameter and type of feed)
Antenna polarisation (linear or circular).
Sun noise (if known)
Any data will be useful - especially if you have a calibrated S-Meter. If 
not, I will assume 3dB per S-point. This seems to be representative of many 
rigs and will probably be close enough. The S-meter should be reading 
something off the end stop with the antenna pointed at clear sky (say S1 to 
S5).
Even if you cannot detect transponder noise, the level of the beacon above 
receiver noise will still be useful.
All inputs will be acknowledged and a summary of the information published 
here when the study is complete (hopefully early in 2002).
73
Charlie G3WDG 

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