Rich, While there have been a lot of good suggestions thrown at you, you are fighting an uphill battle without knowing the spec's that were obtained when the duplexer was re-tuned the last time or finding someone with a tracking generator or network analyzer to verify the duplexer tuning.
Simple method to eliminate coax and/or antenna as possible source of your desense is to tune it up with a good VHF dummy load. I make the repeater work as good as possible running into a dummy load before I connect it to an antenna. If you discover that as others have stated and I also have found out the same results, that your duplexer may not have enough isolation, I recommend adding a 12" pass cavity between the transmitter and the duplexer as the first step. If you are still a little short on isolation, then the addition of another 12" pass cavity between the receiver and the duplexer may be necessary. I always try the cavity on the transmitter side first. For this you need a true pass cavity, one that has two connectors and no notch adjustment. Motorola, GE, and DB-Products for starters have made these 12" pass cavities since the 70's. These pass cavities can often be found used for a reasonable price. 600 KHz split repeaters can be a challenge. I personally like to see a little over 100 db of isolation, especially if you have a decent preamp added to the receiver. Kevin is dead-on about how clean tubes can be compared to solid state PA's. I can run our UHF Micor tube repeaters at 150 watts with a Angle Linear preamp on the receiver, with less isolation required from the duplexer, than the same Micor repeater with a 75 watt solid state PA requires for no desense. For example, I have a Motorola Micor 2 Meter 147.250/147.850 repeater running in my garage on my test duplexer, which is a Sinclair 6 can that has about 94 db of isolation. I have the transmitter set at 60 Watts. With no preamp, I have no desense and some headroom before any desense would occur. With the factory 12 db gain Micor preamp, I have no desense, and a little headroom before desense would occur. With a Angle Linear or ARR preamp with higher gain, I have about 5 db of desense. To eliminate the desense would require another 5 db of isolation from the duplexer, which would put it at just about 100 to 102 db of required isolation from the duplexer. Good luck and let us know how you are progressing. Joe - WA7JAW --- In Repeater-Builder@yahoogroups.com, "RichardK" <shutterbug13...@...> wrote: > > Good evening, our club has a Wacom WP-639 four can duplexer as part of our > repeater system. Input Fq is 147.915 and Output Fq is 147.315. We have a > 600kHz (+) offset. Very simply, our main problem is when we run the > transmitter at full power 100 watts, there is a HUGE desense on the receive > side of things. When we drop the transmitter power level to around 20-50 > watts, the receive side opens WAY up to a large area where people can get > into the repeater. As we begin to bring up the transmitter power, "white > noise" begins to appear and the receive side starts to desense again. All > the cables have been switched to double sheilded cables and all the same > wavelength in length. We have the duplexer seperated & sheilded from the > transmitter & preamp parts. We have not replaced the antenna feed coax with > double sheilded coax yet. Antenna is a Hustler G7 atop a 55' mast. The > duplexer was retuned just over 1 year ago. Any suggestions as to what we > could look into next? Some of us believe the problem is with the tuning of > the duplexer receive cans. Thank you very much. >
