You won't do it with band pass cavities unless you crank the insertion loss way up. What would work for the two simplex radios with the 660 Khz spacing is a pass/notch duplexer. It needs to work just like a regular repeater duplexer with the same kind of isolation from one radio to the other. You need in the neighborhood of 70 to 90 db depending on how good the radios are. Very hard to get with pass band cavities. Just look at the curve of a pass band cavity at different insertion loss settings and move off of center frequency 660 Khz and you will see how far down the attenuation is. Most cavity manufacturers have those curves on their web site. Look at different size cavities, 4", 6", 10" and you will see that the larger cavities are better (have steeper skirts) than the smaller ones. Most will show curves for two cascaded cavities too. You will quickly see what is needed for pass cavities to get the isolation needed.
When you get the repeater frequencies you will probably have to start over again. Two antennas would be a big help if you can do that. 73 Gary K4FMX > -----Original Message----- > From: [email protected] [mailto:Repeater- > [EMAIL PROTECTED] On Behalf Of John B > Sent: Thursday, October 04, 2007 12:51 AM > To: [email protected] > Subject: [Repeater-Builder] duplexer isolation and reciever 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. > > > > > > > > > > > Yahoo! Groups Links > > >
