James, It would be helpful to know if you are planning using cavities and or filtering to help provide the needed isolation.
Also knowing what the receiver will be and what the preamp is if any will all play into what is needed to get any real world numbers to determine the required isolation. In my experience with VHF split antenna repeaters, at that power level and using some pretty good filtering, I would expect that you will need in the neighborhood of 80 to 100 feet between the top of the bottom antenna and bottom of the top antenna. This just a guess not knowing the answers to the first two questions. Sounds like a fun project. I prefer a split antenna repeater anytime over a single antenna duplexed repeater. We use 225 Watt Micor UHF repeaters for our ham stuff here on the Oregon Coast. I love the challenges of making the high power stuff work. Joe - WA7JAW --- In Repeater-Builder@yahoogroups.com, James Adkins <adkins.ja...@...> wrote: > > We are considering installing a 2-meter repeater, standard 600 kHz spacing, > with separate antennas for transmit and receive, looking at phasing together > 2 DB-228's for RX and 2 DB-228's for TX and using a high-power transmitter, > such as a Motorola Nucleus at 250-300w or other high-power transmitter. > > Does anyone have a formula or know what formula would need to be used to > determine the amount of vertical separation needed to provide the isolation > required for such a duplex operation? > > We are wanting separate TX and RX antennas because of plans to have the > repeater on a platform located 1200' in the air, and heliax runs are not > practicable. > > -- > James Adkins, KB0NHX >
