Dave VanHorn wrote:
>I have a choice on where I apply a bandpass filter in the system, and
>I don't see a clear winner..
>
>On a 2m repeater, I have a preamp after the cans. I'd like to add a
>filter in front of it.
>
>Now I could put the filter between the receive cans and the amp.
>The filter then only acts on the input signal.
>
>It occurs to me that I could put the filter between the cans and the
>antenna, which would give me the same filter action for the amp as if
>the filter were behind the cans, but in this position the filter also
>acts on the transmit signal.
>
>Assume that I can tune the filter so that both frequencies are well
>in the passband, and that I don't mind the additional loss in the
>transmit path.
>
>What other effects should I consider?
>
If you have a "window" filter that is wide enough to pass both the RX
and TX, then it's okay to use it between the duplexer and antenna.
If you have a Band Pass cavity, it's "Q" or bandwidth is likely too
small to give acceptable results placing it between the duplexer and the
antenna, and the recommendation is to place it between the RX port of
the duplexer and the input of the preamp with this cavity center tuned
to your receive frequency.
Cavity filters are very selective and the reactance it would create if
tuned to the midpoint of your repeater frequencies and placed between
the duplexer and antenna would likely result in a blown up transmitter.
Plus, the loss it would create on receive would be very detrimental to
the systems sensitivity.
My question on the addition of the filter is... Why do you feel the
necessity to add the filter in the first place?
If you are experiencing a problem, tell us the symptoms. If you aren't
experiencing a problem, then consider not adding the filter as any
loss's ahead of the preamp cannot be overcome by its use.
Kevin Custer
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