[amsat-bb] Re: Eliminating Desensing

[Edward Cole]

Andrew is onto something when he suggests 
different preamp sensitivities to out of band 
interference.  One can also just have front-end oveload of energy on frequency.
I have not had any issues with either Mode-UV or 
Mode-VU with my system where the antennas are 
separated 5-feet on a metal boom.  My preamps 
both sue mgf-1302 GasFets.  Both were special designs and not commercial mfrs.

But I will say that ARR preamps seem to work well 
as do Kuhne Engineering (DB6NT) designs.

The diplexer solution works for out of band 
interference when used in front of the 
preamp.  In band front-end blockage can be 
handled by filtering the transmit coax line 
either with a bandpass filter or 1/4 wave coax 
stub tuned for the receive frequency.

Often it takes a bit of trial and error to find 
the solution for your particular situation.

73 Ed - KL7UW

At 09:22 AM 6/30/2008, Andrew Glasbrenner wrote:
>It's unusual to see desense on 2m receive from a 
>70cm uplink, but I think the same solution can 
>be applied. Many use a common 2m/70cm diplexer 
>as a high pass/low pass filter. In this case you 
>would want to feed the 2m antenna to the common 
>port of the diplexer, and the 2m port of the 
>diplexer to the input of either your preamp or 
>rig if you do not have a preamp. This blocks 
>your transmitted signal from the receiver/preamp 
>input very effectively. 
>http://www.amsat.org/amsat-new/articles/Mode-J/ 
>describes this for 70cm desense issues. Another 
>option is to use a preamp with good filtering on 
>the input. I had issues with Mode J desense with 
>my Icom AG-35 preamp, but have none now that I 
>have switched to SSB Electronics preamps. As 
>always, Your Mileage May Vary (YMMV). 73, Drew 
>KO4MA -----Original Message----- >From: 
>"Reicher, James" <JReicher at hrblock.com> >Sent: 
>Jun 30, 2008 12:50 PM >To: 
>amsat-bb at amsat.org >Subject: 
>[amsat-bb]  Eliminating Desensing > >I got 
>significant de-sensing of my 2 meter downlink 
>from my 70cm uplink at Field Day.  I know a good 
>part of it was due to the short cross-boom 
>(about 4 feet) I was using.  I would like to 
>decrease the de-sense on this set-up before I 
>permanently install it at my home station.  Any 
>and all suggestions are welcome. > >73 de W0HV, 
>Jim in Raymore, MOÂ  >Light travels faster than 
>sound...  This is why some people appear bright 
>until you hear them 
>speak. > >_______________________________________ 
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