Good question.
You will need crystal or ceramic filters to be able to do this. You
will be bound to a very small bandwidth to work in but it works. It
used to be done by some M/M guys. Probably its still being done to hear
very close to the run station for mults.
Also you might want to try using a vertical for one station and a dipole
or some other horizontally polarized antennas for the other. This will
give an addition 30db (ideal) isolation.
I have also seen a very straight dipole aimed at the other in band
antenna so its null was in the others direction. And it worked but the
spacing was 300 feet or so to the other array.
Another trick is to have a station listen on a beverage. For 40m its
not very long and I have set up a 40m station CW with the SSB FD station
about 400 feet away. It worked very well. Put the beverage out as far
away from the other antenna as possible. Matter of fact it worked so
well, the SSB guys wanted their own beverage the next year. They had a
2 ele beam and we smoked them in QSOs.
Jay
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