Hi Nate
Firstly, in the case of your additional bandpass cavity - if cavities
are properly tuned to 50 ohms, the length of 50 ohm coax between them
doesn't make one bit of difference. The problem most people have with
proper cavity/duplexer tuning is that they don't maintain a 50 ohm
load on ALL ports when they tune 'em. So when they're placed into
service, the port impedances are different and the tuning of the
cavity/duplexer changes. Which is why you should NEVER EVER tune
either without at least a 3 dB 50 ohm pad on each of the ports. From
your description, I'm willing to bet you didn't use pads :-)
My other comment (and in my humble opinion) is that if you're using a
properly tuned BpBr duplexer on a low power repeater (the 720 is 25
watts, yes?) and you need additional cavities in the receive side,
you've got bigger issue than simply needing an additional cavity. You
didn't specify what the inteference is but have you done an intermod
study of the site?
IMHO, the additional cavity is a waste of time and effort until you
identify the source of the interference (and made sure your duplexer
is first properly tuned)
Ken
At 05:09 AM 7/26/2007, you wrote:
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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