Finally found some good diagrams for a 3 cavity bandpass filter at Telewave, and it looks like I can get 80db down with 6db of insertion loss using 5" cavities, which may be acceptable as I can make it up at the antenna if needed. Then it seems I can get the last 10db (if not more) by kicking up to a 6" or 8" cavity to steepen the skirts.
http://www.telewave.com/pdf/TWDS-5012.pdf So bandpass CAN be made to work. the question then becomes: Is there a way to do it with less than a dozen cavities ?? Using notches seems to be counterproductive as I would need 2-3 notch cavities per radio per frequency to notch (call it 3 recievers vs 2 transmitters, or 6x3 - 18 reject cavitites) Do I really need the cavities on the repeater transmitter (which will never be used for receive). might not a Wilkinson splitter/combiner do the trick, bringing at least that one transmitter down 20db before hitting the cavities for the 3 recievers Bring on the "other" ideas :-) _____ From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Eric Lemmon Sent: Thursday, October 04, 2007 9:50 PM To: [email protected] Subject: RE: [Repeater-Builder] duplexer isolation and receiver noise budget John, It might be instructive to let the big-name combiner companies make proposals to solve your dilemma. Send a request for proposals to Telewave, TX-RX, and RFS/Celwave to see what they would recommend. Don't try to design it for them; just give them the frequencies, power outputs, receive sensitivities, feedline type and length, and make/model antenna, and let them come up with their own plans. I think you will be surprised that more than one solution may do the job. My gut feeling is that your requirement to use just one antenna may be a killer, cost-wise. I can think of several combining strategies, but I don't think multiple bandpass cavities is going to work. I think you'll need more notches than bandpasses in any viable combining plan. 73, Eric Lemmon WB6FLY -----Original Message----- From: Repeater-Builder@ <mailto:Repeater-Builder%40yahoogroups.com> yahoogroups.com [mailto:Repeater-Builder@ <mailto:Repeater-Builder%40yahoogroups.com> yahoogroups.com] On Behalf Of John B Sent: Wednesday, October 03, 2007 10:51 PM To: Repeater-Builder@ <mailto:Repeater-Builder%40yahoogroups.com> yahoogroups.com Subject: [Repeater-Builder] duplexer isolation and receiver noise budget I'm attempting to design a system that will have a VHF repeater (freqs not yet determined) sharing an antenna with 2 packet radios (APRS on 144.39 and Winlink on 145.05, either of which may be active as a digipeater at any time). I'm currently considering a bandpass-only "quadplexor" to isolate the radios from each other.. each radio running through a bandpass filter tuned to its frequency only (that includes the transmitter and receiver for the repeater), on the theory that it is a lot easier to pass one frequency than it is to reject 3 others. Assuming that none of the transmitters run more than 50w, how many DB down do I need to be outside of the passband to minimize desense for any of the 3 receivers ?? Any other suggestions on how I might handle this hookup would be greatly appreciated. I'm nearing completion my trailer-mounted 40ft crank up tower, and I'm having some problems budgeting space for a filtering system with 12 bandpass cavities without cutting into general cargo space.
