If this is going mobile on a 40 ft crank up I would sure consider separate antennas is possible. You can get into some real challenges, and even with separate antennas you will still have issues to consider. Also you will have a much better idea when the repeater pair is determined. Steve NU5D
Gary Schafer wrote: > 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 >> >> >> >> > > > > > > > > Yahoo! Groups Links > > > > >
