(having email "issues" today, so if this is a dupe, please ignore)
RE: cell mixes and 440 repeater
I had a 440 repeater at a site with no other UHF transmitters for
probably a mile or two. On an adjacent tower was a cell site (this was
back in the early 90's AMPS days). When certain cell channels were
active I would get a mix product that fell on or near my receive
frequency, manifesting as either feedback "squealing" or sudden
increases in noise levels that made it sound like desense. My
transmitter was a Micor driving a GE 1/4 kW tube amp, the duplexer was a
4-cavity Antenna Specialists pass/reject, and rx was a Micor with an ARR
GaAsFET. I found that putting a harmonic notch filter (one of those
little tunable Celwave jobs) on the output of the duplexer, tuned to the
center of the cell site "B" carrier transmit band, got rid of all of the
problems. I didn't investigate further to determine if the mix was
happening in my Tx, in the preamp, or the Rx.
Before spending money an isolator (which will also require a harmonic
filter after it), you might try a cheap test using a shorted
quarter-wave stub tee'd into the feedline at the output of your duplexer
to see if it makes any improvement. I'd suggest using a piece of 1/2"
Heliax for the stub.
If you have a spare pass cavity (a real pass cavity, not pass/reject),
you might experiment with it on the tx leg of your duplexer, and then on
the rx leg, to help determine if you're experiencing a mix in your tx,
or in your rx, or perhaps neither. Keep in mind that what appears to be
an overall increase in the noise floor might actually be a mix involving
wideband digital cellular (e.g. CDMA).
FWIW, I've had substantial (and that's an understatement) problems with
Henry SS amplifiers being unstable on VHF, UHF, and FM. I've also
received amplifiers from Henry that had the wrong low-pass filter in
them - an FM amp with a LPF cutoff around 210 MHz comes to mind. I also
had a UHF 200 watt ham-band amp that was shipped with NO low pass filter
in it. When I called the factory to complain, I was told that Part 97
had no spectral purity specifications for anything operating above 225
MHz so they didn't bother with a filter. The second harmonic was only
about -30 dBc. Eventually they took the amp back and put in an LPF.
Take a real close look at the spectral output of the Henry while it's
operating into the antenna system before spending any substantial time
or money trying to fix a non-existant problem elsewhere in the system.
--- Jeff
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