I think you need to identify what your priorities for this project are. If it's not cost, then there are several ways to do this thing. It seems like your biggest concern might be physical size of the cavity package.
If that is the case, what I would do would be to get a repeater pair in the 147MHz range, the upper meg of 2M. If that is possible you could get two 2M duplexers, one for the 147 repeater, and another for the 144/145 frequencies. Now you have everything combined into two antenna ports, one for the 144/145 stuff and one for the 147 repeater. Next, you need a way to combine these two ports into one antenna. This could be done with several notch type cavities, or a wideband pass type duplexer. The duplexer solution would be easier, and take less rack space. There is a company called DCI that can build you a custom BP duplexer that would cover the 144/145 on one port and 147 on the other. Should be able to make it with 60 to 70dB of isolation between the two ports, and about 1.5 to 2 dB of insertion loss. Now depending on the duplexers that you choose, it should all fit on less than one standard 6ft rack, maybe even half a rack. You should end up with 75+ dB of isolation from any port to any other, and probably about 3 to 3.5 dB of insertion loss, which is a little much, but acceptable for this type of operation. The bad news is the cost. This could be in the 6K range, give or take, maybe as little as 3K, if you can shop around for the 2 duplexers, and are not overly concerned about the size. Contact www.dci.ca and tell them what you are trying to do, and what they could engineer a solution for the wideband duplexer part of this. They probably can't do anything for the 2 close spaced pairs, and that is where Telewave, dB Products, Sinclair, TXRX, etc will come in. It should work out OK, but using two antennas would be simpler and cheaper, but maybe that isn't an option. Joe --- John Barrett <[EMAIL PROTECTED]> wrote: > 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: Repeater-Builder@yahoogroups.com > [mailto:[EMAIL PROTECTED] On Behalf Of Eric Lemmon > Sent: Thursday, October 04, 2007 9:50 PM > To: Repeater-Builder@yahoogroups.com > 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. > > > > ____________________________________________________________________________________ Be a better Globetrotter. Get better travel answers from someone who knows. Yahoo! Answers - Check it out. http://answers.yahoo.com/dir/?link=list&sid=396545469
