Dave, The vast majority of duplexers will work pretty well with the cables close to the right length. After all, Sinclair offers only two cable harnesses for their Q-202G BpBr duplexers- one for the 136-150 MHz split, and the other for the 150-174 MHz split. I may not have the splits correct, but I do know that there are only two harnesses.
You can't simply cut the jumper cables to a certain fraction of the wavelength, because the coupling loops inside the cans are part of the total length. It takes a network analyzer to determine the optimum length, and most folks simply don't feel that it's necessary to go to that extreme. When you order a duplexer from one of the major manufacturers, the bench technician will select a jumper off a rack of jumpers that are arranged in (usually) 1/4" increments. He uses a network analyzer to measure the insertion loss between two cans at a time, and then he will try a longer or a shorter jumper until he finds the optimum. Needless to say, he soon has a list of "sweet" lengths to start with for each frequency, so it normally takes only a few tries to get it on the nose. It is still a "cut and try" approach, but it works. The typical spectrum analyzer works in transmission mode, and doesn't have the extreme reflection sensitivity needed to properly tune for bandpass. A good spectrum analyzer with a return-loss bridge can probably be used in place of a network analyzer for this task. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Dave VanHorn Sent: Friday, January 20, 2006 7:17 PM To: [email protected] Subject: [Repeater-Builder] Re: Duplexer retune / recoax > The length of the jumper cables between the cans has a profound effect upon > the insertion loss at the pass frequency, and relatively little effect upon > the isolation at the notch frequency. Which brings up a fun question.. How do you know what the right length is, and where do you measure it from? Or is this a cut and try thing? In that case, anyone know sources for bungee coax? :) Yahoo! Groups Links 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/
