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@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of John B Sent: Wednesday, October 03, 2007 10:51 PM To: Repeater-Builder@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.
