Stu, I think 250 watts is far too much power for that duplexer to properly isolate. Try running just your exciter, or try no more than 30 watts or so. My 220 repeater runs just 18 watts, and it is almost perfectly balanced. I am using the same Telewave duplexer, and I have zero desense.
According to my CommShop program, you need more than 90 dB of isolation for 250 watts TX and 0.25 uV RX- and that's assuming a tube amplifier. The Telewave TPRD-2254 duplexer is spec'd at 85 dB, so it is borderline, even when perfectly tuned. You might try a sharply-tuned bandpass cavity to clean up the transmitter output, to see if sideband noise is causing the desense. Also, try a bandpass cavity on the RX input. As has been explained many times on this list, a BpBr duplexer has practically no bandpass effect, and what little effect there is, is very broad. 73, Eric Lemmon WB6FLY -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Stu Benner Sent: Friday, January 25, 2008 9:52 AM 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
