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Re: 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.

The problem is generally the 2M fundamental signal mixing in the front end
of the 70cm receiver and generating the 3rd harmonic there.  So the fix is
to put a 70 cm band pass filter in the 70cm antenna input.  That is what I
did here and it worked great.  It rejects the 2M signal from getting into
the 70cm radio.  I tried a 2M filter in the 2M antenna input and it did
nothing to cure the problem.

Find an older ARRL Handbook.  The later ones don't show it.  I have a 1973
handbook and the filters are on page 499 under the heading Filters for VHF
transmitters in the chapter, INTERFERENCE WITH OTHER SERVICES.  There is a
filter for 6M, 2M and 70cm.  I built the 70cm one.  It is built in a Bud
CU-2113-A minibox, 1 1/8 x 2 x 10 inches.  Very simple and very effective. 

> 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.
> Regards,
> Steve Diggs, W4EPI
> AMSAT Area Coordinator, Atlanta
> EM73WU
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73, Roy

Internet: w0sl@amsat.org
Home Page: http://home.swbell.net/rdwelch
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