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March 7th 04, 08:15 PM

On 7 Mar 2004 03:12:05 -0800, (Mark Keith) wrote:

"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.

Myself, I would probably feed it from the base. One problem often seen
with sleeve dipoles like that is decoupling the feedline from the
antenna. if you do center feed it, pay close attention to feedline
decoupling. MK

Hi Mark,

That is the point of a sleeve dipole (and the design of the Isopole)
in that the quarterwave section below is the decoupling.

I've plaid a variant upon this by attaching a ground wire to a point
on the shield, down and away from the feed point. It requires that I
strip away a portion of the coax outer dielectric to do this, and I
use a wire connected to a BNC female jack as the radiator portion (the
line is a conventionally terminated BNC-BNC coax). This way I can
rummage through combinations of lengths and ground or non-ground
situations to suit the purpose (usually field day variations). I
prefer off-center fed designs.

73's
Richard Clark, KB7QHC