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" [email protected] >Sent: Jun 30, 2008 12:50 PM >To: [email protected] >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. > >_______________________________________ ________ >Sent via [email protected]. Opinions expressed are those of the author. >Not an AMSAT-NA member? Join now to support the amateur satellite program! >Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb _______________________________________________ Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb