[Moon-Net] FT8 for EME

Joe Taylor joe at princeton.edu
Thu May 21 16:47:05 CEST 2020


Hi all Loonies,

Some here will be interested to learn that earlier today W2HRO and PA2V 
easily made (as far as I know) the first EME QSO using the FT8 mode.

Paul and Peter used WSJT-X 2.2.0-rc1, a beta-release candidate for 
version 2.2 of the program WSJT-X.  Both stations have moderate 4-yagi 
setups on 432.  Conditions today were not particularly good: degradation 
around 3 dB, and the Sun only 20 degrees from the Moon.

For terrestrial use the FT8 decoder searches over the range -2.5 to +2.4 
s for clock offset DT between transmitting and receiving stations.  When 
"Decode after EME delay" is checked on the WSJT-X "Settings" screen, the 
accessible DT range becomes -0.5 to +4.4 s.  Just right for EME.

FT8 uses 8-GFSK modulation with tones separated by 6.25 Hz.  At the time 
of this QSO the expected Doppler spread on the W2HRO - PA2V EME path was 
8 Hz, which causes some additional loss of sensitivity.  Nevertheless, 
as you'll see in screen shots posted here, copy was solid in both 
directions:

https://physics.princeton.edu/pulsar/k1jt/W2HRO_PA2V_432_EME_1.jpg
https://physics.princeton.edu/pulsar/k1jt/W2HRO_PA2V_432_EME_2.jpg

Why might you want to use FT8 instead of "Old Reliable JT65" for EME 
QSOs?  FT8 is about 4 dB less sensitive than JT65, but with 15-second 
T/R sequences it's four times faster and it doesn't use Deep Search. 
When I was active in EME contests on 144 MHz, I was always frustrated 
that even with reasonably strong (for EME) signals, one's maximum JT65 
QSO rate is about 12 per hour.  With FT8 you can do 40 per hour, as long 
as workable stations are available.

What about FT8 EME on 1296 MHz?  It might sometimes work, but Doppler 
spread will probably make standard FT8 a problem.  But if there were 
sufficient interest, we could make an "FT8B" or "FT8C" with wider tone 
spacing.

Please try FT8 for EME on any of the bands 144, 432, and 1296 MHz, and 
let us know your results.

	-- 73, Joe, K1JT


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