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MICROWAVE COMMITTEE COMPONENTS SERVICE
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AO-40 Signal / Noise Spreadsheet
A spreadsheet has been developed to enable users to calculate the signal to noise ratio for the AO-40 beacon, the amount of transponder noise that should be seen and what improvement might be expected in increasing dish size (relative to a 2 ft dish). The reason why a pro-rata improvement in performance with increasing dish size is not obtained with a satellite where a transponder is in operation is that the satellite itself radiates noise power as well as signals. Larger dishes pick up more of this noise, which adds to the receiver's own noise and causes a diminishing improvement in S/N as the dish gets larger. This is unlike "normal" operation where you would expect a doubling of the dish diameter to give a 6dB improvement in S/N, all other things (like feed efficiency etc) being equal. The diminishing return gets greater with lower noise systems, with shorter satellite ranges and low squint angles, and these effects can be analysed with the spreadsheet. However, there will often be circumstances where poor squint at longer ranges will mean that the larger dishes will give useful improvements! When measuring signal or noise levels, my preference is to use a calibrated S-meter. The S-meter on my IC735 was calibrated using an HP signal generator with known variable output level. This indicated approx 2-3dB per S-point, which may be typical of many modern radios. Measurements made using this method have been cross-checked using a spectrum analyser and good agreement was obtained. The system gain should be such that the S-meter reads something off the end stop (eg S1-2) with no signal and the antenna pointed towards clear sky. This also ensures that the system has enough gain to overcome the noise from the station receiver, and I believe this is good practice for ultimate receiver sensitivity when dynamic range is not an issue, as with AO-40. Other assumptions in the spreadsheet are a) an antenna temperature of 30K (this can be changed), b) that loss due to squint follows Fig 7 of the paper by VK5HI on the AMSAT-NA website [curve fit performed up to 35 degrees only], c) dish aperture efficiency of 45%, and circular polarisation. The spreadsheet is based on signal to noise ratios measured off-air from AO-40 on 16 Dec at MA=101 with a squint angle of 15 degrees, using two dishes of 5ft and 10ft diameter. This data enabled as estimate to be made of the beacon eirp and also the eirp of the transponder noise (measured 10kHz below the beacon). It has been noted to me that transponder noise level may well vary somewhat according to the particular circumstances and so the predictions may only turn out to be a rough guide. As more data is collected, this should become clearer. Until the spreadsheet has been tested more extensively, please do not rip your equipment apart if you do not achieve the kind of levels it predicts! Large discrepancies may indicate a system problem, but may also arise from inaccuracies in measurements, or this spreadsheet. Any feedback would be welcome! The spreadsheet can be downloaded here. [The file was scanned for viruses using Norton Anti-Virus software and signature file from 19 Dec 2001 and found to be free from viruses]. For safety, please re-scan the file before use with your own AV software. No intentional use is made of macros. Note: with some browsers, a right click may be needed to have the option of saving the file to disk without running the spreadsheet. Description version 1.2 dated 29 December 2001 Spreadsheet 1.1 issued 20 Dec 2001: initial version |