Topband: FT-8 performance

[K4SAV]

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