Topband: FT8 - How it really works

[K4SAV]

While sitting around being bored and recovering from a gall bladder 
operation, I decided to do some experiments with FT8.  First thing I did 
was upgrade the software to WSJT-X v2.0.

I hope this post doesn't turn into another FT8 bashing session. My only 
goal was to understand how this mode works and what it can do and what 
it cannot do.

The official description of FT8's signal reporting cannot be correct.  
It is obviously not a signal to noise number and it is not an S meter 
reading.  What is it? That was the first question to answer.  It's 
obviously not an S/N number because how do you give a report of -1 dB 
for a signal that is S9+40 dB on a quiet band.  I was unable to find any 
info on how the signal report was calculated so I tried to correlate 
those reports to observations.

I think I have figured out a method that results in very close to the 
same number that FT8 reports.  Here is the experiment.  I set up my main 
VFO to USB 2500 Hz bandwidth and set the second VFO to CW at about 150 
Hz bandwidth.  I look for a station calling CQ and tune the second VFO 
to him and measure his signal strength.  I also look at the S meter for 
the signal level on the main VFO.  I also look at the signal report 
calculated by the software.  For stations calling CQ that report is 
calculated by the software in my computer.

The FT8 report is usually very close to the difference in signal levels 
(VFO1 - VFO2).   For example if the main VFO reads S9+10 and the second 
VFO reads S9, the FT8 number will be -10 dB.  Note that the FT8 says 
that -24 dB is the lowest it can decode.  With VFO1 = S9+10, that's 
about S7 for the smallest signal it can decode.  Observations agree.  
Those numbers will vary a little depending on how your S meter is 
calibrated.  In order to decode a weak signal, all those close USA 
stations will have to go silent.

The official documentation for FT8 says it will decode signals 24 dB 
below the noise floor.  That is not a correct statement most of the 
time.  That statement should be that FT8 will decode signals 24 dB below 
the sum total of everything in a 2500 Hz bandwidth. If the total of all 
signals on the band are below the noise floor, it would be interesting 
to know if FT8 will decode any of them.  I haven't observed that yet in 
a real situation.  I did however try to simulate that condition by 
adding enough noise to the signals such that all the signals were below 
the noise.  The software did continue to decode signals.  All the 
reports were -24 dB.  This was a very crude test because I don't know 
how exactly much the signals were below the noise.  This should be of 
benefit to those people that have S9+ noise on the bands they operate.  
They should be able to decode the strongest signals on the band.

The (VFO1 - VFO2) test just described should always result in a number 
equal to or less than zero.  I notice sometimes the software will report 
a small positive number.  That seems to happen more often when the 
bandwidth is set to something less than 2500 Hz and there are very few 
signals on the band.  I think this may be related to the fact that FT8 
does all its calculations using audio signals and the receiver S meter 
is operating on RF. Audio shaping in the receiver will affect the FT8 
calculations. Audio processing in your computer sound card may be a 
factor too. This becomes really apparent when the radio is set to CW and 
the audio peaking filter is turned on.  With SSB bandwidth and flat 
audio response, S meter readings are a good indication of what will be 
decoded.  It should decode signals down to 24 dB below whatever your S 
meter reads.

I also narrowed the bandwidth of VFO1 and chopped out a bunch of 
signals.  I got S7 on VFO1.  Then a station calling CQ also measured S7 
on VFO2.  The FT8 report was 0 dB.  Agrees.`

That test brings up a possibility.  If you can narrow VFO1 to a very 
narrow bandwidth hopefully containing only a very weak signal, then you 
may be able to decode it.  A strong signal in the passband of VFO1 will 
kill the decode.

It works.  I decreased the bandwidth of VFO1 to 200 Hz and it decoded an 
S2 signal.  I had VFO1 in USB mode with that bandwidth. My receiver will 
go to zero bandwidth in USB mode.  I put VFO1 into CW mode at 100 Hz 
bandwidth and it decoded a signal that was moving the meter between S0 
and S1.  That signal would have also been easy copy if it was CW instead 
of FT8.  I was using a good receiving antenna on 160 meters immediately 
after sunset.

While this seems to work for weak signals it is a non-starter for normal 
operation.  How do you tune around with a very narrow bandwidth looking 
for a station calling CQ or any other station that might be DX?  It's 
not like CW, unless you learn to copy FT8 by ear.  You can't find him 
with a wide bandwidth because the software won't decode him.  He is only 
there when the bandwidth is very narrow.  Given the number of USA 
stations on FT8 that bandwidth will have to be really narrow to keep the 
USA stations out of the passband.  Even 50 to 100 Hz bandwidth usually 
doesn't do it on a crowded band and you can't go lower than that and 
still decode the signal.  This doesn't sound like anything that is 
practical.  Maybe something useful might be to improve the copy of a 
weak station by narrowing the bandwidth if you already know the station 
is there.

One thing you could do is set the receiver to a narrow bandwidth and 
call CQ DX, listening only on your transmit frequency. However the DX 
station would probably need to be receiving with a very narrow bandwidth 
or he won't hear you because you are probably very weak on his end too.  
I seriously doubt that he knows to do that because it seems that no one 
else knows about that either.  Besides it is not often that a rare DX 
station will respond to a USA station calling CQ DX.  Another 
non-practical suggestion.

There are DX stations strong enough to be decoded that can be worked 
with FT8, especially on the higher bands like 20 meters. Even on 160 
meters sometimes a DX station will be strong enough to be decoded.  Just 
tonight right after sunset I heard a couple of European stations on 160 
running S5 to S6.  Because they were so strong, I tuned down to the CW 
portion of the band but I didn't hear a single signal from anyone down 
there.  Oh well.

Seems to me that FT8 is a very poor method of working weak signal DX.  
It also seems that it isn't being used that way either.  Just listening, 
it seems that everyone is working very strong signals, 20 to 40 dB above 
the noise floor, at least as observed at my station.  Maybe this isn't 
the case for people that have an S9+ noise floor.  For those people, if 
they can't reduce the noise, FT8 may be the only way they can do any 
operating.

At least I now know more about FT8 than I did before starting this 
exercise.  Learning stuff is never boring and it killed some time, and 
my big incision feels a little better.

Jerry, K4SAV