Cable lengths sometimes are critical and sometimes not, depending what you are trying to accomplish with the particular setup. In the case of interconnecting cables between notch cavities a critical length cable will provide more notch depth because it unloads the next cavity.
In the case of using pass band cavities to make a combiner into a common junction the cable lengths are critical even though everything is running at 50 ohms. The off frequency skirt of the cavity provides a short at some frequency and that short needs to be transformed to a high impedance at the junction so as not to load the other devices. DUPLEXER TUNING: In this case proper duplexer tuning should make little if any difference in the interference problem noted. As long as there is no desense from his own transmitter the duplexer is doing what it can. A bp-br duplexer has little off frequency rejection (pass band rejection) compared to a pass cavity. The bp-br duplexer does provide a little pass band help but not as much as some people like to believe. The frequencies between the transmit and receive frequencies have the most rejection from pass band effects but outside of them there is little. As was stated he is getting interference from a public safety transmitter so it would fall outside the more protected part of the duplexer. An additional pass filter on the receiver would indeed provide more protection but 45 db sounds like a lot to expect from a pass filter unless the frequency is a long ways away. As Skip noted a notch cavity at that point may well provide more protection. More to the point of cable length, a pass filter actually looks like a notch filter at the unwanted frequency i.e. a low impedance at that frequency. So ideally if you want the most rejection a cable that provides a half wave length at the unwanted frequency will reflect that low impedance provided by the cavity skirt to the next port in the system at that frequency. In this case the difference will probably not be noticed for the trouble involved. With the public safety antenna right beside the repeater antenna there can be fundamental overload of the receiver so some form of additional protection may help. 73 Gary K4FMX _____ From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Ken Arck Sent: Thursday, July 26, 2007 9:40 AM To: [email protected] Subject: Re: [Repeater-Builder] Coax length between added cavity and duplexer 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 >> -- 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 <http://www.debian.org/> ---------------------------------------------------------------------------- -- President and CTO - Arcom Communications Makers of repeater controllers and accessories. <http://www.arcomcontrollers.com/> http://www.arcomcontrollers.com/ Authorized Dealers for Kenwood and Telewave and we offer complete repeater packages! AH6LE/R - IRLP Node 3000 http://www.irlp.net <http://www.irlp.net/> "We don't just make 'em. We use 'em!"
