Oh yes, I remeber that! it was dubbed G-Line from its inventor, Georges
Goubaut, canadian I think.
that line has very low losses and can operate wonderfully. However, in
this case it's going to be hard to couple the cavity to such a line.
the Gunnplexer is phase sensitive and needs circular polarized antennas
to operate correctly. I would suggest that a better approach would be
to waterproof the whole thing and locate it on the tower, going down
with cables on the low frequency side. I used to peel off the mica
window found on old kerosene burning heating stoves and ose that with a
few dabs of RTV to waterproof the cavity opening and put the whole on
the roof.
Saandy 4Z5KS