At 1/17/2010 01:13 PM, you wrote: >Greg, > >Trying to combine both vertical and horizontal separation will not help; the >horizontal will completely swamp the vertical. As soon as you move the >transmit antenna out from directly under the receive antenna, the isolation >decreases dramatically. Even a few feet of horizontal displacement will >have a major influence. > >For example, CommShop recommends 86 dB isolation for your system with a 20 >watt transmitter and a 0.25 uV receiver. That isolation can be achieved >with about 190 feet of vertical separation, one antenna directly above and >in line with the other, or about 16,200 feet of horizontal separation.
There's a 2 meter repeater about 3000' from my home running ~25 watts. I just checked to see if there's any measurable desense caused by that repeater to my FT-8500 radio 600 kHz away. I do see ~ 1 dB - barely noticeable. The desense is in the form of front-end compression, so any decent RX like a Mastr II or Micor wouldn't be bothered at all. Adding a modest amount of pass cavity filtering to the TX & RX can dramatically reduce the amount of antenna separation needed. My very first repeater used 2 antennas separated 50' horizontally with a single pass cavity on the TX. The transceiver was all tube & the RX antenna was a beam nulled into the TX antenna. I had to retune the TX about once every 2 weeks & keep the sharp null of the Yagi on the TX antenna, but when this was done there was no desense. Bob NO6B
