[amsat-bb] Re: Arrow antenna reconfiguration results - UPDATE

Mike Ryan mryan301 at comcast.net
Sat Sep 26 08:33:44 PDT 2009

I've had my battles with desense. Originally I was going in the direction that the RX antenna was in the envelope of the TX. It made sense and some elmers suggested it. Even at low power (5-10W)  and separating antennas by 30 ft it would still de-sense, but not as pronounced (omnidirectional antennas). 

I fixed it two ways. The biggest poroblem was my linear. To be kind to my FT857 I ran it at low power through a mirage amp to get output up to 35 watts or so max. 90% of my problem was that amp (which I assume was pushing sprurious or harmonics). If I took it offline and ran the FT857 at the same 35W the desense was greatly reduced but not quite gone. The remainder was taken care of by using a second diplexer as a poor mans low pass filter. Just put a dummy load on the side I didnt need. 

Now the omnis can be close without this problem and I even built a pair of cheap Jpole yagis on the same boom (definately in the same envelope) but no desense. I never got around to trying another amp. I was just delighted to have the problem solved.

So there's my story in case it helps someone else consider this possibility instead of spending all the hours chasing it that I had to endure!


-----Original Message-----
From: amsat-bb-bounces at amsat.org [mailto:amsat-bb-bounces at amsat.org] On
Behalf Of Joe
Sent: Friday, September 25, 2009 6:43 PM
To: Gary "Joe" Mayfield
Subject: [amsat-bb] Re: Arrow antenna reconfiguration results - UPDATE

as in the texts below,  there is something else going on here.

That Diplexor can not be all that bad. two reasons.

How many db down is the front to side of that antenna?

and I can not imaging someone would sell a diplexor that has greater 
than 20 db of losses.

because of the statement that how criticalpolarity was with the 
original, and now the antenna has to be nearly 90 degrees cross 
polarized to make it drop out  uhh

that close to 30 db,

at least 20,,

something else is going on here

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