Nate, Was the interference present before the duplexer was retuned? If not, then I suspect that the tuning is not correct. Although I applaud your ingenuity in the duplexer tuning setup, a proper tuning of the notches on a BpBr duplexer really needs to be done on a spectrum analyzer or a network analyzer. The typical BpBr duplexer has a very broad peak that can be tuned precisely only with a network analyzer or a spectrum analyzer with a return-loss bridge. When tuned for return loss, the bandpass can be tuned with great precision. A network analyzer also has the advantage of presenting precise source and load impedances to the cavity being tuned, which makes it easy to tune them individually for cascade connection. Separate matching pads are not required with such an instrument; the match is built-in.
How many cans are in your Wacom duplexer, and what diameter are they? You might find it useful to employ the interference calculation procedures found in GE Datafile Bulletin 10002-2: <www.repeater-builder.com/ge/datafile-bulletin/df-10002-02.pdf> Also download the interference analysis worksheet here: <www.repeater-builder.com/ge/datafile-bulletin/df-10002-03.pdf> The above documents were only recently added to the GE Master Index, and have great potential value in this instance. As for your original question, I believe that cable length between the additional bandpass cavity and the duplexer output should not be critical if proper tuning procedures are followed to ensure 50-ohm source and load impedances. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Nate Bargmann Sent: Thursday, July 26, 2007 5:09 AM To: [email protected] Subject: [Repeater-Builder] Coax length between added cavity and duplexer 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 intermittently. 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 >>
