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Anything else >> General and other topics >> Antenna gain vs C/N https://www.satsig.net/cgi-bin/yabb/YaBB.pl?num=1303243692 Message started by radiofm on Apr 19th, 2011 at 9:08pm |
Title: Antenna gain vs C/N Post by radiofm on Apr 19th, 2011 at 9:08pm
Hello
I have a question about relationship between antenna gain and C/N level. example: antenna 1.2 meter gain 41.1 dB = C/N on spectrum analyzer 14 dB. when i change this antenna to 1.8 meter with gain 44.6 dB (same location, LNB, antenna pointing to max signal) so C/N will be better about 3.5 dB ? and i can recive on spectrum analyzer C/N 17.5 dB ? Best Regards Robert |
Title: Re: Antenna gain vs C/N Post by Eric Johnston on Apr 19th, 2011 at 11:00pm
Assuming you are measuring a carrier and not a satellite originated CW beacon.
The downlink C/Ndown will improve by 3.5 dB but the overall C/Ntotal will not improve by that full amount. The C/Ntotal you measure on the spectrum analyser is the summation of C/Nup + C/N intermodulation (if multicarriers in the transponder) + C/N down. For example: C/Nup= 25 dB C/Nintermodulation = 21 dB C/Ndown = 15.5 dB. C/Ntotal = 25dB + 21 dB + 15.5 dB = 14 dB If you improve the C/N down by 3.5 dB so C/N down = 19 dB the C/N total becomes 16.2 dB, which is only 2.2 dB improvement. To do the calculation yourself do the following with the input C/N values: divide by 10, change sign, 10^X. Write down the amount. Do this for each value and add them all up. Process the total thus: log, then times 10. e.g. 25 dB > 0.00316 21 dB > 0.00794 15.5 dB > 0.02818 total = 0.03928 > 14 dB 25 dB > 0.00316 21 dB > 0.00794 19 dB > 0.01259 total = 0.02369 > 16.2 dB Typically a VSAT outlink carrier or TV carrier from a large hub dish to the remote sites will operate with a high uplink C/Nup and the C/Ntotal at the remote sites will be closely associated with the VSAT dish size. The high uplink C/Nup is achieved using a large dish, high power amplifier and a low gain setting on the transponder. Return links from remote sites are likely to have their C/Ntotal constrained by the small VSAT size and transmit power. The satellite transponder has to operate at a high gain step setting. The result is that the C/Ntotal depends mainly on the uplink C/Nup and increasing the hub receive dish size from say 6m to 13m to improve the dowlink C/Ndown makes no significant improvement overall. Play here: https://www.satsig.net/linkbugt.htm wxw Best regards, Eric. * The above analysis is simplified as it does not mention the uplink and downlink interference entries from cross-pol and adjacent satellites, which might typically be 25 and 25 dB. These figures vary significantly. |
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