Joe,
Even if your FR-4000 is the correct split (450-470 MHz) for GMRS, it may
require tuning of the receiver's front end before being placed in
service. I learned the hard way, when I set up some Kenwood TKR-750/850
repeaters, that sometimes there are helical resonators in the receiver
that must be optimized. The Icom unit may have a similar design in its
front end. Check the service manual to see if such adjustments are
required.
Even if no such adjustments are required, the FR-4000 may simply be
incapable of avoiding desense from nearby transmitters- even if they are
far removed in frequency. If the VHF transmitter on the same tower is
on a frequency that falls in or near the range of 155.850-155.908 MHz,
its third harmonic will likely desense your repeater's receiver- even if
it is within the FCC specs for spurious emissions.
You did not mention what type of duplexer you have, but if it is the
mobile notch variety, that may be part of the problem. Since GMRS is on
UHF channels with a 5 MHz split, mobile notch duplexers work just fine
in most applications. The problem with mobile notch duplexers is that
they have no pass selectivity at all; the only adjustment is for the
notch. Even pass/reject cavity duplexers have only modest bandpass
selectivity.
Assuming the FR-4000 receiver is properly tuned, it may be necessary to
add one or two bandpass cavities between the duplexer and the receiver
to tighten up the selectivity.
73, Eric Lemmon WB6FLY
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