Re: [amsat-bb] AO-40 Downlink Interference

[Bob Johnson <stest034@xxxxxxxxxxxxxxxxxxxx>]

On Monday 21 October 2002 10:22 am, Jesse Morris wrote:
>[...] 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.
>

This is sort of a long shot, but:

Perhaps you aren't pointing at the source of the other signal, but at the 
source of the mixer that is combining and re-radiating them.  E.G. a 
corroded metal-metal connection in an old nearby tower or building.  When 
you are pointed at the signal, are you pointed at any sort of nearby 
structure?  This would assume at least one of the signals is fairly strong, 
but your 435 signal probably qualifies for that.

When I say "nearby", I'm thinking yards, not miles.

> 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.
>

In general, your description sounds like some sort of radar, but that's 
a pretty wild guess.  The weather service's NexRad doppler radar operate 
at around 2.7 - 3 GHz, so you could check to see if one of them  started 
operating near you a few months ago, but they take several seconds to 
scan a complete circle, instead of the 1/2 second or so modulation you 
are hearing.  That sounds more like some sort of approach control radar 
(any airports in that direction?).  Or, of course, it may not be radar at all.

> Any suggestions would be appreciated.
>
> 73 de Jess - W4MVB

- Bob
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