At 8/30/2008 11:52, you wrote: >Tom, > >According to my CommShop duplexer isolation calculation software, a 600 kHz >split at 2m with a 50 watt transmitter and a receiver with a (my assumption) >sensitivity of 0.35 uV, about 87 dB of isolation is necessary to achieve no >desense. If you intend to use two antennas, be aware that they will need to >be separated by about 202 feet vertically or over three miles horizontally. >Neither of these separations is practical for a portable repeater.
One thing to consider is that since the primary limiting factor is TX noise getting into the RX, adding a pass cavity on the TX dramatically reduces the amount of separation required. I once operated a 2 meter system with a single pass cavity on the TX & ran the RX feed straight to the RX antenna, only 60 ft. about the TX antenna. Worked fine with no desense for several years. Another option would be to go with a low phase noise PLL exciter; see the G.E. duplex curves links posted by Dave KA9FUR a couple of days ago. Still not practical for portable use; IMO standard 600 kHz split systems are impractical as portable repeaters, as by definition that can't be truly "portable". >Perhaps you should consider a UHF portable repeater, which can use a notch >duplexer for the 5 MHz split. However, the ordinary UHF mobile notch >duplexer will normally not tune down into the Ham UHF bands without having >the internal coupling loops modified at the factory. Wow, that's the first time I've ever heard that. I've bought probably over a dozen UHF mobile duplexers over my lifetime & never had a problem tuning them down to 440-450 MHz. Bob NO6B
