I never built this, but had considered an antenna like that. I was going to
patch in a 1/4 wave section of triaxial coax. The two shields would be
connected at the top, and the outermost shield would be left unconnected at
the bottom end. BTW, I hung a 40m 1/4 wave ground plane vertical from a tree
branch once. Feed point was at about 30 - 35 feet. There were 4 radials
sloping down at 45 degrees. It worked reasonably, but unfortunately came
down before I had a chance to do much with it. The sloping radials will give
you very close to a 50 Ohm impedance.

Tam/WB2TT
"Alex" wrote in message
le.rogers.com...
Has anyone tried building a 1/2 wave coaxial sleeve antenna for 40m? I
have
a large tree with a branch at 70' and i thought i'd pull the antenna up
with
a rope. Should i make the sleeve out of copper pipe? A 1/2 wave vertical
would be a nice antenna for 40m and i thought a coaxial sleeve antenna
would
give me a great feed impedance. Any thoughts? The coax are 100' of RG8.