E: Since I have much experience with blowing coax relays or preamps, I constructed this circuit. It has push-pull outputs, which can drive a PTT to ground or a +12V as a direct supply for a preamp. Its delay times are configurable as well as the output polarity and the delay on TX start or RX start.
Only one transisitor of the output stage shoudl be equipped at the same time, whatever is needed, push-to-ground or +12V-output.
Picture: Place Plan
Picture: Solder Side (click on the image to get a Postscript file)