I'm helping a club in the next county get their repeater working
better. A couple of weeks ago they brought the RF unit (TKR-720) over
and we (KC0MLS, K0BYK, and myself) checked it out. The PA transistor
required soldering and after that everything checked out well.
Next we checked out the duplexer, a Wacom BpBr set. Lacking a tracking
generator, we used our ancient IFR-1200 and a reprogrammed Spectra
mobile radio and tuned the pass filters for best SINAD and the notch
filters for the poorest SINAD for their respective frequencies.
After they put everything back on site, it all works well except that
the local public safety is getting into the receiver intermintently.
My first thought was intermod, but the various programs don't turn up a
match for the receiver's frequency.
A week ago we were able to visit the site and tightened several loose
connectors on the other hardware in the site. Since then the
interference does seem to be less but is still present on occasion.
Observations of the site revealed that the public safety and the club's
repeater antennas (DB-224 style, unsure of exact models) both share the
top of the tower and are broadside to each other and are maybe four
feet apart at most. So now our thinking is that the problem may be
receiver overload.
We set up a spare Celwave bandpass cavity that has about 2 dB of
insertion loss and offers about 45 dB of insertion loss at the public
safety's transmitter frequency. My question is whether the coax length
is critical between the RX port of the Wacom duplexer and the input
port of the Celwave cavity? I plan to send along a length of RG-393
(double shielded teflon coax) with the cavity. As far as I know, it is
a random length. Should I cut it to something closer to 1/2
wavelength? 3/4 WL?
Thanks!
73, de Nate >>
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