> There seems to be confusion & mystery in the crowd of radio
> buffs that I han out with (not in this group!) on the subject
> of connecting cable lengths for cavity & duplexer use. I've
> heard that the manufacturers of such devices also tend to be
> mum on this.
>
> It seems to me that if these devices are tuned up to perform
> their function(s) (notch, peak, bandpass, bandreject, etc.)
> with 50 ohms of pure R in & out as source & load Z, cable
> length ought not make a difference (within reason, I'm not
> talking long cables with significant losses) to their performance.
>
> Is this so? Am I missing something? Thanx!
You're on the mark. Another one of those topics that comes up here pretty
often. If everything is built and tuned correctly to present a load Z of 50
ohms, the interconnect cable lengths don't matter. Duplexers and cavity
filters have critical-length interconnect cables as part of their harnesses,
and those lengths ARE critical, but the lengths of the cables going to/from
the repeater should not be critical if the duplexer is tuned such that it
gives a high return loss at the respective pass frequencies at the
input/output ports.
Unfortunately, some duplexer manufacturers instruct users/techs to tune
pass/reject duplexers for least insertion loss without any consideration of
the return loss at the pass frequency, which often results in a poor match.
I don't know whether they just assume that nobody has a means for measuring
return loss, or they just figure that tuning for least insertion loss will
get you "close enough for field work". Every manufacturer that I know of
factory-tunes tunes cavity filters on a VNA looking at both S11 and S21, so
why they would recommend something else for field-tuning seems like bad
advice to me.
For your friends that like to quote magic formulas like "the cable length
between the transmitter and duplexer should be an even half-wave", ask them
from where should you start measuring this magic half-wave? From the
transistor collectors? From the combiner that combines the final
transistors? From the input of the low pass filter? From the output of the
low pass filter? From the input to the T/R relay or the output of the T/R
relay (if it has one)? From the antenna jack on the back of the amplifier?
I'd be curious to hear the answer :-)
--- Jeff WN3A
