[amsat-bb] Overcoming desense on FO-29

Alan wa4sca at gmail.com
Sat Mar 21 11:39:43 UTC 2015


The problem is caused by overload of the RX preamp, and possibly front end of the 70 cm rig.  You will
still have the (same) preamp, and the converter has its own front end.  So no, that will not likely do
any good.  Getting as much separation between the antennas, a good preamp, using the mode-J filter
which Drew mentioned, and running minimum power are your best bets.  With a good mode-J filter, SSB
preamp, and 10' between the uplink and down link antennas, I can run 50 watts TX and barely hear any
hiss.  If you are using linear antennas, using different orientation between TX and RX will give you a
few more dB of isolation.

People do use your suggested arrangement with a TS-2000 to avoid the downlink birdie on SO-50.



<-----Original Message-----
<From: AMSAT-BB [mailto:amsat-bb-bounces at amsat.org] On Behalf Of Jim Barbre
<Sent: Friday, March 20, 2015 6:51 PM
<To: AMSAT-BB at amsat.org
<Subject: [amsat-bb] Overcoming desense on FO-29
<Is the use of a down converter effective in overcoming desense on the
<FO-29 70 cm downlink? The downconverter would convert 435 to 10 MHz.
<Receiver is an SDR. (I have different ones for HF and for VHF.)
<Also, what is the minimum recommended distance between linearly
<polarized 2 m and 70 cm antennas on the same crossboom? Does it help
<with desense if one is mounted horizontally and the other vertically?
<Jim Barbre
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