How are you checking for desense? Are you using an isolated T between the duplexer and antenna line and doing the same when measuring desense on the dummy load?
Are you measuring site noise? Do this the same way you would measure desense with the isolated T in the line. But first see what the receiver sensitivity is with the dummy load connected in place of the antenna. Then replace the dummy load with the antenna, do not key the transmitter, and measure the difference in receiver with the antenna connected verses the dummy load. With TV stations present you may be surprised at the amount of site noise present. Then key the transmitter and again measure receiver sensitivity thru the isolated T. If you haven't done this you may find that a lot of the problem is site noise rather than desense problems. 73 Gary K4FMX _____ From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Stu Benner Sent: Friday, January 25, 2008 12:52 PM To: Repeater-Builder@yahoogroups.com Subject: [Repeater-Builder] Desense Problem on 222 MHz Repeater Our group has substantial technical knowledge and experience, but we've been just about beaten down by a problem with our repeater. A brief overview of our situation follows. We have a 222 MHz repeater comprised of a converted Micor mobile, Telewave TPRD-2254 BpBr duplexer, AM-6155 PA modified for class C operation at 250W, and a DB-264JJ antenna at 80 ft. fed by 1/2' Heliax on a commercial FM broadcast tower . With the duplexer terminated into a load, we have about 1 dB degradation in sensitivity when transmitting. However, with the antenna connected to the duplexer, we experience in excess 15 dB of desensitization. We have eliminated other narrowband transmitters and analog TV transmitters as contributing factors. We are left with a channel 12 digital TV transmitter at an adjacent site as a key contributor to the problem. Our hypothesis is that we have broadband IMD products from the mix of our transmitter and the DTV transmitter that are appearing in and near our receiver passband. Is it a "rusty bolt" problem or is there some other non-linear component somewhere on the site or in our system that is the mixing point - we don't know. I'd be interested in beginning a dialog with anyone who might be able to give us some further insight into this problem. Regards, Stu Benner W3STU Boonsboro, MD
