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Old 7-Oct-2015, 10:34 AM   #2
Antennas Direct Tech Supp
Join Date: Jan 2010
Posts: 2,942
is there EVER a need for that antenna's output to be run through an FM trap filter?
Yes. That long boom picks up a LOT of FM, especially when situated at or near a right angle to the boom's aiming direction.

Does that possibility change if it's filtered through a standard UHF/VHF UVSJ combiner, with the vhf side capped?
Yes, the UVSJ would be expected to attenuate everything, including FM, below the UHF port's cutoff frequency by a minimum of 20 dB. It would take an extraordinary level of FM signal to require a second stage of filtering.

Or if the UHF antenna is pre-amplified?
I've learned the hard way that it's usually better to keep FM out of your pre-amp unless you have a very robust amp that won't overload or produce spurious signals.

A spectrum analysis would be needed to pin down specifics, so you're usually left with following "best practices" if you want to side step those issues.
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