Ian, the simplest solution I know of is one I have used some years ago on
144MHz. The antenna is hard-wired to the transmitter, and fed to the
receiver via a 1/4 wavelength of co-ax  from the transmitter, with a pair of
back-to-back diodes from the co-ax centre to ground at the receiver end.
When transmitting, the diodes turn on and effectively short the co-ax to
ground. This is reflected back to the transmitter as an open because of the
1/4 wavelength of co-ax. As soon as the tx stops, the short goes away and
the rx can hear again. The diodes can be any high-speed switching diode (we
used to use 1n914, but they are obsolete now) that will handle a few
milliwatts..

 Cheers, Ken,
 vk7krj@southcom.com.au

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