Eric,
Thanks. Your reply is very helpful. I especially am encouraged by your suggestion that the pass cavity be re-tuned and put on the TX side. That makes good sense to me, and I have equipment adequate to do that job. I don't have a network analyzer or spectrum analyzer with return-loss bridge. I have to rely on friends and benefactors when I need such equipment, and I don't want to go to that well too often. I doubt very much that we are getting total notch of 85 dB. The transmitter and receiver are the Yaesu Musen FTR-5410. The 5 watts out feeds a Mirage solid state PA. I don't have the PA model number at hand. The PA gives about 90 watts out. I do not have a manual of any kind for the PA, so I do not know whether there is a better operating point. I would gladly reduce power some if it is controllable and would give a cleaner signal. Thanks again. John Transue -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Eric Lemmon Sent: Wednesday, August 20, 2008 12:33 AM To: [email protected] Subject: RE: [Repeater-Builder] De-Sense and Circulator Question 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: Repeater-Builder@ <mailto:Repeater-Builder%40yahoogroups.com> yahoogroups.com [mailto:Repeater-Builder@ <mailto:Repeater-Builder%40yahoogroups.com> yahoogroups.com] On Behalf Of John Transue Sent: Tuesday, August 19, 2008 1:09 PM To: Repeater-Builder@ <mailto:Repeater-Builder%40yahoogroups.com> yahoogroups.com 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. __________ NOD32 3192 (20080616) Information __________ This message was checked by NOD32 antivirus system. http://www.eset.com
