I'm responding to you directly, as well as to the list, because your
question is a very good one that should be answered publicly.
Without much fear of contradiction, I will state that a
"bandpass/bandreject" duplexer has precious little bandpass effect. Sure,
when you look at a response plot that covers 10 MHz, it may look very
selective, but in reality the bandpass action is rather subtle. A whole
lot of RF can blow through a Bp/Br duplexer in either direction, but MOST
of the time this deficiency doesn't cause a problem.
I would not put a BP cavity between the duplexer and the antenna, simply
because such a cavity would diminish the function of the duplexer. Now,
if you want to put a preamp between the duplexer and the receiver, a BP
cavity tuned to the receive frequency is a very good idea. This BP filter
should follow the duplexer and precede the preamp. Two 8" cavities with
1.0 dB IL work well.
If you have a solid-state PA that has significant sideband noise, you
might consider placing a BP filter between the PA and the duplexer. If
you are using a ferrite isolator to protect the PA, a BP cavity filter
between the isolator and the duplexer can take the place of a 2nd-harmonic
notch filter or low-pass filter. At a crowded commercial site, you may be
required to have a BP cavity on the TX side.
73, Eric Lemmon WB6FLY
w7aor wrote:
> How common is it to use a band pass filter ahead of a pass-reject
> duplexer, i.e., between the duplexer and feed line to antenna. Any
> reason for adding a band pass filter there on antenna input to duplexer
> vs. between the preamp pre selector cavities receiver port of
> duplexer.
>
> Please respond to w7aor @narri.org
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