Eric Lemmon WB6FLY gave the best response to my question of "how
common is it to place a bandpass fillter betweeen antenna and
duplexer. Most assumed a ham repeater with tx circulator and high
rejection pass/reject duplexer which is correct.
As I read the resposes it seems the answer is: this is not very
common at all and not the best way to lower a nose floor to increase
Rx sensitivy. It also adds to Tx power loss by a few dB. If broad
enough to pass both Tx and Rx then the skirts are too broad to be
helpful in eliminating RF from near by out of ham band high power
transmitters.
The repeater owner said he allowed a radio shop to install it that
way, but it did NOT do the job of lowering the noise floor as
promised. I questioned the wisdom, but thought I would ask if others
had do it that way and why.
--- In Repeater-Builder@yahoogroups.com, Eric Lemmon <[EMAIL PROTECTED]>
wrote:
> 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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