The idea of just pulling the braid back over the outer jacket of the
coax to make a 1/4 wave section turns out to not be a great one. The
reason is that you want 1/4 wave considering the propagation velocity
of the coaxial arrangement consisting of the feedline outer conductor
and the folded-back braid, with the (generally vinyl) coax outer
jacket as dielectric. The result is a "line" with not very well
controlled VF around 0.5 (so you should use only about 1/8 freespace
wavelength of it, resulting in a very off-center-fed dipole). If you
make it a quarter wave in freespace and it does have VF around 0.5, it
will be close to an electrical half-wave and reflect back...a short,
where you wanted an open! Also, the higher the impedance of that line
is, the higher an impedance it will reflect to the lower end where the
feedline comes out, and the close spacing and high-dielectric-constant
jacket conspire to make a low impedance line, which is not good. So
the idea of using a cage of wires forming a narrow cone should be a
very good one. That's a cage version of an Isopole, I guess. Wire's
pretty cheap; a cage of 8 conductors should do a good job (and be
lighter than a sleeve made of copper pipe!). I'd try to make the
bottom end of the cage at least a foot in diameter, with the coax
nominally centered in it. If you do use a pipe, try to use an ID as
large as practical and keep the coax more-or-less centered in it.

Cheers,
Tom

"Alex" wrote in message ble.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.