Tom--- I don't think that your feed line is the problem----sounds like corrosion in the antenna---is this a possibility?
What can happen is that your forward power into the antenna sets up significant current in the antenna structure ( and near tower structure too) which is rectified when and if it passes through corrosion. The rectification (some call it micro-arcs) results in noise that is wideband enough to cover the repeater's receive frequency and this goes back down the feed line and through the duplexer to cause desense. The interesting thing is that you often, or maybe I should say usually, can't hear any change in receiver output noise between transmitter on and off---just desensing of an external signal. Have I had this experience?-----Yes. In my case, a 2 meter Hustler went bad---received fine with the transmitter off and showed good VSWR with it on but extreme desense and the wideband desensing noise was visible using an iso-T and spectrum analyzer at the receiver input. What was the cure? Took the antenna down and overhauled it and put it back. No more desense for several years now. Scott, N6NXI ----- Original Message ----- From: Tom Elmore To: [email protected] Sent: Friday, October 03, 2008 12:10 AM Subject: RE: [Repeater-Builder] LMR-400 Cable Here is the latest. I terminated the cable at the antenna with a dummy load and no desense. I hooked back up to the antenna and if listen to the receiver with the squelch open I hear a buzz in the background of the receiver white noise audio in addition to desense which doesn't show up when terminated. Checking the swr I have less than 1/2 watt reflected with about 80 forward. The offset for this receiver is 1.7 Mhz by the way. If I take a 1/4 wave mag mount antenna that I have sitting on top of the duplexer cabinet and plug it directly into the receiver I hear no buzz or desense. This tells me something is going on with the antenna and coming back the line. Is it time to put in some hardline? Tom / KA1NVZ -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] Behalf Of [EMAIL PROTECTED] Sent: Thursday, October 02, 2008 8:47 PM To: [email protected] Subject: Re: [Repeater-Builder] LMR-400 Cable At 10/2/2008 05:41, you wrote: >Hello Tom, > >I think that the clue here is that you had desense with both the RG-213 >and the LMR-400. My guess would be that there is some mismatch between >the antenna and the duplexer. The duplexer may be tuned for a 50 >resistive ohm load, but the antenna system is presenting some other >impedance/reactance. Neither RG-213 nor LMR400 are suitable for duplex use, so it's quite likely that both were the source of the desense. Bob NO6B
