Agreed the duplexer most likely will not be pure anything. Seems like Wacom tuned the pass/reject with a sweeper, then looked at return loss. Odd quarter wavelengths of cable would transfer the Z at the duplexer to the antenna, transmitter or receiver, but most PA's are broad banded and could not tune into the imperfect load anyhow. I don't know what would be gained varying the cable lengths between the repeater and duplexer. Hopefully from the tee to the antenna is a given, but again, I doubt the output of the duplexer is exactly the same as the line to the antenna, but I doubt you could have much improvement doctoring cable lengths. Of course there is stub tuning, and double stub tuning, etc. Me thinks the losses would exceed the gains, Your mileage may vary, Have a great weekend, Steve NU5D
[EMAIL PROTECTED] wrote: > 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 > >
