Topband: Transmit antenna de-tuning relay.

[Greg Chartrand]

 The best way to isolate the TX antenna for receiving is to have it float above ground. The worst means is grounding the antenna. The grounded antenna will re-radiate any noise it captures rendering your RX antenna useless. It only takes a few PF to ground to make a TX antenna your principal noise source so keeping that in mind, deploy the relay(s) such that the antenna sees no ground.
In my case, I have a center loaded vertical, it connects to a L network matching circuit; the isolation relay opens the wire going to the vertical so it floats well above ground. It can get tricky is you have a grounded shunt fed tower, however, I found floating the shunt wire works as well as anything.
The best test for finding the best isolation strategy is to take an AM radio tuned as high as it can go and put it in as close as you can to the vertical. You should hear the re-radiated noise make the radio blast noise. Now you can test grounding (if you don't believe me), floating,and even de-tuning. BTW I could never get de-tuning to work as well as floating.
Good luck!Greg W7MY