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Mode J Recieve Desense Problems

During 1999, I have managed to get my station into reliable Mode J
operations and have enjoyed working many of you on AO-27. Station layout
is a FT-736R, SSB preamps on 2M and 70CM, Cushcraft beams controlled by
G-5400 setup, KCT, and Nova software. Works great. Except for the 70CM
desense I get when I key up. Bummer. I gather this problem is old as
time. Also, beams are mounted 8 feet apart on a fiberglass rod.

Research so far has taken me through the ARRL satellite experimenter's
handbook, ARRL Handbook (1991), DCI web page, SSB tech support, and
various friends. All mention the problem with only vague suggestions for
fixing it. Most seem to point to a low-pass filter on the transmit side
to eliminate the third harmonic of 145.85 from swamping the SSB preamp.
DCI has one, but the darn thing costs $360 smackers. Does anyone know a
less expensive 2m low-pass filter for sale? The Handbook seems to
suggest that one can build a Butterworth filter...and they supply the
engineering info in chart form to do it. Has anyone built one?

I am opening this topic up for general comment. Anyone have alternate
suggestions for fixing 70cm desense? Also, does the same problem happen
in reverse in Mode B?

Thanks for all assistance. I haven't seem this topic discussed in the
last couple of years of monitoring AMSAT-BB.

Steve Diggs, W4EPI
AMSAT Area Coordinator, Atlanta
Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
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