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More JT44 on Ao-40..



Hi All,

Well, my adventures on Ao-40 continue. I have been doing more testing with
the Jt-44 weak signal program on Ao-40 whenever the squint angle is too high
to work any ssb, or probably even cw.

I found that one difficulty is knowing where your downlink signal will be in
relation to your uplink signal. Since the  normal practice of just tuning
for your own carrier doesn't work too well ( cause you can't hear it, hi ),
I had to find a way to reliably tune in the signal. Then I remembered
Spectran, that wonderfull piece of dsp software from out Italian friends. It
will run at the same time as JT-44, so I could tune in my signals even when
they were below audibility.

Well, it works exactly as advertised. I was able to find my own signal
fairly quickly, and from there the JT-44 frequency display got me down to
the last few hertz. The JT-44 sync tones seem to be around 1300 hz, so by
tuning the rx, and watching Spectran for the train of pulses at about 1300
hz, I could quickly find myself, and as a bonus, it would be a good way to
identify a JT-44 station that you couldn't hear. The pulse train is quite
unique, with the higher frequency tones showing up as random dots nect to
the sync pulses.

I think this would be an ideal way to find JT-44 stations operating at a
defined frquency on the satellite, such as 50 khz below the beacon. This
"calling frequency " idea is used for JT-44 on the vhf/uhf bands, but there,
the frequency is stable. On the satellite, the first chore is figuring out
where you are, hi.

One thing I did notice in my testing. It seems that it is easier to decode
very weak JT-44 signals when I am using the L2 uplink rather than the L1
uplink, even with the 6 db weaker signals on L2. I think this may be due to
the way the L1 receiver jumps back and forth in frequency every 10-15
seconds. The 10 hz movement it makes is probably enough to upset the
decoding algorithm in JT-44. Since the frequency on L2 is rock stable, it
seems to decode somewhat better.

So far, I have worked 2 other stations successfully on JT-44. I also had
another station try to answer my CQ, but he was too far off in
time-synchronization to decode, and I didn't know how to adjust for that at
the time. But, I do now, hi. I believe it was a JA station, but never got a
confirmed callsign. This was at fairly high squints of about 45-50, when
there where no other signals to be heard.

I seems that this mode might be quite interesting for those times when
regular communications are too difficult, or for small stations that  want
to play. I have decoded my own signals under good conditions ( less than 20
degrees squint ) while running only 40 watts eirp on the L-band uplink, so
it opens up some interesting possibilities, I think.

That's all for now, folks....see you on there.

regards,

Roy VE7BPB
Vancouver


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