Ian, >From your description, I can think of two reasons why the duplexer is not performing to your expectations: 1. It is, in fact, NOT tuned correctly, or 2. It is incapable of performing properly in the new band segment that you are tuning it to.
Many years ago, I bought a Yaesu/Vertex VXR-5000 UHF repeater and the matching Vertex notch duplexer from a major Amateur Radio supplier. When I tested the duplexer on my bench, I found that it had been tuned for minimum insertion loss at the pass frequencies, rather than for maximum rejection at the notch frequencies. Once I tuned it correctly, it worked fine. But, I guess the dealer's technician just didn't understand how a mobile notch duplexer works- and he probably didn't read the instructions. You did not state whether your duplexer was for VHF or UHF, but I know that most UHF mobile notch duplexers have relatively narrow bands of operation, so it is not always possible to move such a duplexer from the commercial band down to the Amateur 70cm band, for example. A Celwave PD633-6A-2N duplexer works well in the 450-470 MHz band, but cannot duplicate the performance of the PD633-6A-1N duplexer in the 406-450 MHz band where the Amateur 70cm segment lies. The reason for that is because the coupling loops are factory-set for a specific band, and require special tools to adjust.. When a mobile notch duplexer is properly tuned in the band for which it was designed, the insertion loss at the pass frequency should be around 1.5 dB, which means that you should get around 28 watts out for 40 watts in. The rejection at the notch frequency should be at least 70 dB. I have not found the cable lengths to be critical in the dozen or so repeaters I have set up, that use mobile notch duplexers. That is not a claim that cable length is irrelevant, but merely a statement that I personally have not experienced any impact from cable length- or perhaps I'm extraordinarily lucky! Yes, it is possible that the duplexer has been damaged by lightning or abuse- running excessive TX power, for example. Finally, it is possible that a previous owner inverted the tuning because he thought that the side marked "TX" had to be connected to the transmitter even when the new repeater was low in-high out rather than the more common high in-low out arrangement. To avoid that misunderstanding, most duplexer manufacturers now use "pass low, reject high" and "pass high, reject low" terminal markings. Please provide some information about your duplexer, such as the brand and model number, and what frequencies it was previously tuned to as well as what the new frequencies are. When you tested the duplexer at its original frequencies before making any tuning adjustments, did it perform to factory specifications? 73, Eric Lemmon WB6FLY -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of kerinvale Sent: Friday, August 28, 2009 7:09 AM To: Repeater-Builder@yahoogroups.com Subject: [Repeater-Builder] Cavity/duplexer connection cables Hi again guys .After tuning a notch mobile duplexer with a service monitor and spectrum analyzer .The tuning indicated the duplexer was tuned correctly but when the 40 watt transmitter was applied then the output of the duplexer was around 10 watt then certainly something is wrong with the duplexer.I wonder if the low output of the duplexer is not bad matching of cable length but is more tied up with a faulty duplexer.maybe a burnt track of carbon inside the tuning tubes? Any thoughts .I have tuned 6ld450s notch duplexers with no worries and they produce 20 watt or more from 40 watt in. Thank You, Ian Wells, Kerinvale Comaudio, 361 Camboon Road.Biloela.4715 Phone 0749922574 or 0409159932
