[amsat-bb] AO-40 Downlink Interference

[Jesse Morris <w4mvb@xxxxxxxx>]

Good Morning,

Three or four months ago I developed what I thought was a severe case of
cross modulation interference between my 435 Mhz uplink and 2.4 Ghz
downlink.  The nature of the interference was a broad spectrum of birdies
every 6 or 7 Khz all across the downlink band when I transmitted on 435. 
My first assumption was that I had gotten water in one or more of the
many connectors and cables in my 2.3 Ghz quad helix antenna system.  When
I took the rotors down for repair a couple of months ago I also took down
the quad helix and replaced it with a single 21 turn helix that I had
used before.  I also replaced the TSI 3733 converter with a different one
and installed a new DEM preamp.  Just for good measure I replaced all the
rusted and corroded U-bolts and hardware with new SS hardware.  When I
got everything back together the problem was worse!

Yesterday I went through the 453 Mhz system with a fine toothed comb and
did not find any problem.  Late in the day while I was resting my tired
and skinned knees (my antenna change-over relay and the polarity
switching system is located in the attic!) I was listening to the
interference while transmitting a very low level (less than 1 watt) of
435 power and realized that the interference had a modulation associated
with it.  The modulation was pulsing at perhaps 2 pps and was a sort of
raw buzz that I sometimes hear on HF and 2 meters.  I started playing
with the antenna position and finally realized what I that was hearing
was an actual received signal - but I only hear it when I'm transmit on
435 Mhz.  It comes from a specific direction to the SW and from the
horizon.  The nature of the signal is a spectrum of pulses about 300 Khz
wide.  As luck would have it I only hear the signal when I am
transmitting in the AO-40 435 uplink band and listening on the 2.4 Ghz
downlink.  I assume it is a received signal that is mixing with 435 Mhz
transmit signal.

Has anyone else heard anything like this?  If it is a legit communication
signal, say in the 1.9 or 2.8 Ghz range, I'm not sure where that leaves
me.  My 3733's have the front end filter mod so I guess I had better put
away all the other projects on my work bench and get busy with the 1.2
Ghz uplink project.

Any suggestions would be appreciated.

73 de Jess - W4MVB

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