John, I don't think a circulator will do much for desense, since its primary purpose is to protect the PA from being damaged by an incorrect load impedance. Of course, it will greatly reduce the chances of IM interference.
The Motorola T-1504A duplexer is barely adequate for a 90-watt transmitter, especially if it has a solid-state PA. You may be able to adjust the loops slightly to increase the notch depths at the cost of increased insertion loss. Use a network analyzer, or a spectrum analyzer with a return-loss bridge, to tune the pass rods for maximum return loss (minimum insertion loss) and tune the loops for a total notch of at least 85 dB. Another suggestion is to put the bandpass cavity on the transmitter- after re-tuning it to the TX frequency, of course. This will make a huge difference if the desense is caused by wideband noise generated by a solid-state PA. Some of that noise might fall right on the RX frequency, and a bandpass cavity on the RX frequency won't help you at all in that case. A circulator with the reject load resistor included is called an isolator. Telewave offers a discount for Hams, and you can get a T-4530 UHF single isolator for about $425. Go here for the data sheet: <www.telewave.com/pdf/TWDS-4015.pdf> It would be helpful to know the make and model numbers of your transmitter and receiver. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of John Transue Sent: Tuesday, August 19, 2008 1:09 PM To: [email protected] Subject: [Repeater-Builder] De-Sense and Circulator Question Members of Repeater Builder have been very helpful in my effort to improve the operation of a 440 repeater. The de-sense problem is reduced now by the addition of another cavity, a band pass cavity on the receive leg. Now I am wondering whether a circulator would provide additional isolation and further decrease the de-sense problem. Your advice would be much appreciated. I have no idea where to get a circulator or what to expect it to cost. If you think a circulator would help, perhaps you can suggest sources and prices. Thanks. John Transue P.S. Additional info: The repeater runs about 90 watts into the duplexer. The duplexer is a Motorola T-1504A. The separation is 5 MHz. It operates in the ham band.
