That is interesting that both you and Coy mentioned the z-matcher. I have never heard of one. I will be looking into it for the sake of knowledge enrichment ;-)
I assumed that the input cable length from the pa to the duplexer did not matter and that I hit some kind of magic wavelength length that botched it up. Adding the three inch longer cable fixed/changed it. Since I now measure -1.1db through the duplexer I'm happy that I got it all matched right. I'd feel sorry for a guy trying to figure this out without a tracking generator to tune the duplexer. That made me feel confident enough that the problem was somewhere else. I considered retuning the duplexer under power to peek it, but decided against it. That would have worked around the problem or made things worse. It was a very interesting learning process this week that started with desense and ended with a new duplexer and TX cable. At the start was 15 watts out of the duplexer with 15db desense. Now its 32 watts out and 0 desense. Think they will notice the difference? ;-) -Tim -----Original Message----- From: Eric Lemmon [mailto:[EMAIL PROTECTED] Sent: Friday, October 01, 2004 6:55 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Duplexer story update Tim, Your experience is a reminder that repeater construction is not an exact science! The fact that the power through the duplexer changed radically when the jumper cable length was changed is evidence that your power amplifier was not correctly matched to your duplexer. Although the length of the cable is certainly a factor, you should also check to be certain that the cable is not defective. If you have several hours of free time, and nothing else to do, you can use the "cut and try" method of finding the magic length of cable that transforms the output impedance of your PA to the input impedance of your duplexer. Simply fabricate a number of jumper cables that are cut to length at 1/2 inch intervals, and try each one until the forward power is at a peak. Some proponents of this method prefer to start with a cable that is a foot or so too long, and then cut off 1/2 inch at a time, reattach the connector, and retest until the optimum length is found. It is difficult to establish the exact length a PA to TX cavity jumper must be, even if the velocity factor of the cable is known, because its length also depends upon the length of the coupling loop inside the cavity- and the loop length may be an unknown. If you don't have a lot of idle time to play this game, you can purchase an impedance matcher (Z-Matcher) for about $90 that will allow you to perfectly match your PA in about 30 seconds. These nifty devices are made by Telewave, Sinclair, and others. They are nothing more than a pi network inside a box, tuned with two variable capacitors. You could also build one of these devices for much less money, but then you would be spending a lot of time building something that is supposed to save you a lot of time. It is possible that a ferrite isolator between your PA and the TX cavity input will improve your power throughput, but that device should be employed for PA and IM protection more than PA matching. YMMV... 73, Eric Lemmon WB6FLY Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
