I'm not sure how FT-8 calculates the reported S/N number. I found very
little information on the subject and what I did find was not easily
understandable. What I did was an experiment in which I was able to get
close to the same number being reported. According to what I have read
about FT-8, it does not implement the same method as I was using in my
testing.
It was very obvious to me that the number being reported was useless.
.Example: How would you be able to report a S/N of -1 dB when the
station is S9+40 db on the S meter and the receiver reads S1 when tuned
to a spot with no stations. (Actual measurement)
I made a guess that the number being reported was actually a signal to
noise plus signal ratio S1(S1 + N), where N is the sum of everything
else in the passband. The S9+40 db station in the example would be the
main contributor to the overall level of the total stuff in the passband
and that total is just a little more than his signal alone, so -1 dB now
makes sense. This seems to work and it works on other FT-8 signals as well.
Jerry, K4SAV
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