I performed S/N ratio measurement on 29/07/2021 using a R&S FSP38 Spectrum Analyzer. I have a 1.2m dish with dual-band coaxial feed fitted with a SPCR5300 LNB (measured noise figure 0.9 dB). An AOR AR5000A receiver with AGC off was used for these measurements. I measured on Rx's IF output – 10.7 MHz.
The channel power measurement for 2500 Hz bandwidth (SSB) was performed. As a signal source I used a middle PSK beacon. For noise power density measurement, I used clear free frequency, 10 kHz below mid beacon. I used an RMS detector, since there is no correlation between the peak value of the video voltage and noise power. The resulting S/N ratio was 16.49 dB with fluctuation a few tenths of dB. For the different bandwidth (BW), for example 1 Hz, the result must be corrected by 10 · log (ChanBW), i.e. 10.log(2500) = 33.98 dB. So, my measured result for 1 Hz bandwidth is giving S/N ratio 50.47 dB. The achieved results very well correspond with Goonhilly and IS0GRB Earth Station S/N values.
As was formerly written by Achim, Peter and others, when transponder noise is dominant over the Rx noise, then there is no room to improve S/N ratio and the above described test is becoming a hearing test or operator capability test, not an equipment test.
73 & BR, Rasto OM6AA