Bruce, The last sentence of your message says it all, and I completely agree that a Z-matcher is the way to go. The original posting was a request for a simple formula to determine in advance how long the jumpers should be to achieve optimum performance. I think we both agree that there are too many variables to have a simple formula.
While the receive-side jumper may work well at any random jumper length, the transmit-side jumper is a different matter. I think we agree that the TX-to-duplexer jumper can be optimized, and many installers do take the time to try several different cable lengths. One installer I know takes a box of a few dozen RG-400 cables to each new install, each cable being cut and connectorized at 1/2-inch intervals from about 18" to about 40". He will keep trying cables until he finds the optimum length. This is just too much work for me! All of those N connectors and the cables aren't cheap, either. Regarding the tuning, I always use a network analyzer to tune a duplexer, and I know that it is optimized for a 50-ohm source and load. The PA is another matter, and it seldom provides a 50 ohm match to the duplexer. That's where a Z-matcher is a valuable tool, but- as can be seen by several dissenting posts- not everyone thinks it is necessary! 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of [EMAIL PROTECTED] Sent: Friday, June 29, 2007 2:20 PM To: [email protected] Subject: Re: [Repeater-Builder] Cable formula Eric I don't think the cable cares whether the source and load impedances are 50 ohms resistive. I think the cable is indifferent to whether the load and source values are resistive or whether they present a complex impedance involving +/- J. as long as the composite value looks like 50 ohms. The conventional wisdom generally expressed is that as long as the cavities are properly tuned, that the interconnect length from the TX is immaterial. I question that: Properly tuned? When what's properly tuned and for what parameter? Is the pass section of the cavity(s) being tuned for maximum output or is it tuned for minimum reflection back to the TX source? Only if the TX output impedance exactly matches the cavity impedance and the impedance of the interconnecting cable will the cavity tuning point be the same for either parameter. To assume that the TX output impedance is 50 ohms is optimistic and as you point out, altering the power level of the TX can affect TX output Z, the amount dependant on what TX stages are used to control TX output. Considering how nit-picky forum members are about designing and building their systems, (and I mean that in the best sense of the word), it seems inconsistent to be indifferent to how the duplexers might be affected by inserting what is potentially a radical impedance transformer between the TX and the cavities. In the absence of any way to measure any source and load mismatch, using a 1/2 wave (or half wave repeating) cable length will at least keep any existing mismatch status quo. It won't improve the match but at least it won't increase a mismatch because the 1/2 wave length simply repeats the TX output Z and does not act as a line transformer. But as the cable length departs from a 1/2 wave and approaches a 1/4 wave, the game changes and a 1/4 wave interconnect between a mismatched source and load can produce some eye opening shifts in the impedance reflected to the load and back to the source. On the other hand, some source and load mismatches can actually be improved by using the interconnect as a line transformer. I spent a lot of hours fiddling around with line stretchers and cut and try cable lengths to determine an optimum length for cables but gave it up in favor of a Z-Matcher which will compensate for any inherent TX mismatch or a shift in power output. Bruce K7IJ
