• S/N ratio is, especially for CW, a function of detection bandwidth, or in the case of an SDR, a function of sampling rate and fft size. For a regular analogue radio with real HW filters, the CW power is (hopefully!) considerably more than the noise, so the (peak) power measurement will fairly accurately depict the carrier energy level with even quite wide detection bandwidth. For noise however, we need to consider the noise bandwidth. The broadband, filter passband-filling noise power (which should be detected in RMS by the way) will be highly dependent on the bandwidth of it, i.e. how much noise energy is being integrated into a number. Increasing the detection bandwidth from e.g. a ~300Hz CW filter to a ~3kHz SSB filter will result in 10dB more (noise) power. In other words the CW carrier S/N ratio will appear 10dB worse with a 3kHz filter than a 300Hz filter

    The point is, to make any kind of S/N ratio comparison, we need to know the modulation (CW is very different from 400BPS PSK) and we need to know the detection bandwidth. And in the case of SDR it is important to know the sample rate and FFT size (bin count). Only in this way can we compare apples with apples or make orange/apple conversion calculations to make comparison possible.

    And for really meaningful S/N measurements, peak detection should be used for the (CW) carrier (S) and RMS detection for the noise (N).

    It does not hurt to realize that the noise factor (N) is often noise + interference (I) and that a more accurate measurement would be S/(N+I), but fortunately for us, due to it's very high linearity, the QO-100 transponder noise floor has been and is pretty much random noise. In AO-7, AO-40 and the like, this was not so simple as HELAPS and other high efficiency RF amplification and other linearization and non-linearity processes resulted in noise-sounding (N+I.)

    The result of all this is that 9dB SNR can easily be the same this as a 25 dB SNR. Only the measurement conditions are different.

    73 - Michael, oh2aue


    "If you have data, you have something, if you do not, you have nothing." (Bengt Hultqvist, SK 24.02.2019)

  • Hi Thank you for answer.the

    I'm using following method for measuring S/N ratio :

    1) I made I^2+Q^2 average it over 1Sec. I and Q samples are collected at 16K sample rate. and after 2.7KHz ssb

    2) I made ratio between reading when I tune the beacon and when I tune just the noise.

    I fully agree with Michael that depend on modulation. Using FFT (That show the peak) I get about 10 db higher.

    Do you know if exist a standard?.

    I also have some tree in front of the dish but the blockage should be vey small.




  • Hi

    After better point of Antenna and lnb position I get becacon (pk with FFT) 21-22Db. That is ok for 40cm dish.

    It's the time to think to the TX.

    I'm thinking to use Pluto..Anyone had experience with with it on NB transponder?.

    Some picture on my GNU radio RX locked on upper beacond and antenna.



  • A 40cm dish has 18 dB gain at 2.4 GHz. Another potential portable uplink antenna is a standard loop yagi which would need to be about 1m long. Potentially easier to fit in travel baggage if it splits into smaller lengths. For RX you still need the dish though so a dualband feed might be the best answer.