Re: [Repeater-Builder] Interference from Public-Safety Station (Was: Coax Length...)
* Eric Lemmon <[EMAIL PROTECTED]> [2007 Jul 26 22:34 -0500]: > Nate, > > Was the interference present before the duplexer was retuned? Honestly, I don't know. It was "off the air" for some time most likely due to the final transistor becoming unsoldered on one lead and the controller being out to lunch. Also, I didn't monitor it to speak of the past several years. So, this is almost like a new problem. > 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. I suppose that stuff could be rented.The other option is the send it to the factory at the mercy of UPS... Seriously, while doing it right is the best way, most of us don't have access to that sort of equipment. We have an ancient IFR-1200 at the shop that is too old to even put a tracking generator in (we've asked). And for the price of the equipment above I could buy many other things that would be far more satisfying including a down payment on a countryside QTH, if one ever comes available. So, we use the SINAD method. Honestly, I don't know what the big deal is as the loss figures matched the specs and we did nothing to disturb the coupling (I *won't* touch that!). We can discuss impedances, but in the real world, there will always be a difference between the test equipment and the devices that are connected together on site. A lot of good information has come from this thread as well as useful ideas. Thank you! But, shelling out 5 to 6 Grand for a couple of dB improvement is not in my budget. ;-) > How many cans are in your Wacom duplexer, and what diameter are they? 4, 8" most likely. > You might find it useful to employ the interference calculation procedures > found in GE Datafile Bulletin 10002-2: > > > > Also download the interference analysis worksheet here: > > > > 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. Thanks for the info Eric (and to everyone else as well). Honestly, I expected a reply or two, but it's been fun to see everyone run with this thing. :-) 73, de Nate >> -- Wireless | Amateur Radio Station N0NB | Successfully Microsoft Amateur radio exams; ham radio; Linux info @ | free since January 1998. http://www.qsl.net/n0nb/ | "Debian, the choice of My Kawasaki KZ-650 SR @| a GNU generation!" http://www.networksplus.net/n0nb/ | http://www.debian.org
[Repeater-Builder] Interference from Public-Safety Station (Was: Coax Length...)
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: Also download the interference analysis worksheet here: 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: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Nate Bargmann Sent: Thursday, July 26, 2007 5:09 AM To: Repeater-Builder@yahoogroups.com 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 >>
