"Tom Bruhns" wrote in message
m...
Think of the coaxial line formed by the pulled-back braid (outer
conductor), the vinyl jacket (the dielectric), and the braid of the
feedline going up (the inner conductor). It's shorted at the top end,
and you want it to be 1/4 wave long so it reflects an open circuit at
the bottom where the feedline emerges. But that's 1/4 wave
considering the velocity factor of that line. And the vinyl jacket
has a fairly high dielectric constant, so the VF in that section may
be about 0.5. If you cut it for 1/4 wave in freespace, that makes it
close to 1/2 wave considering the VF. That's really bad: it reflects
a short instead of an open, and there's no decoupling between the
antenna and the feedline. If you want to do it right, use a tube for
that sleeve whose ID is a few times the diameter of the coax feedline,
and it would be ideal if you remove the jacket and just have a very
few thin dielectric "washers" to keep the feedline centered in the
sleeve tube. Then it's tough to mount! So another way is to make a
rigid feedline (like from copper pipe), maybe 40 inches long, with a
rigid sleeve (which of course is the lower half of the dipole, too)
over it and then you can mount it by the rigid section of feedline.
At about this point, isn't it easier and more reliable (potential
serious safety issue here for a boat, eh?) to just buy a commercial
antenna?

Cheers,
Tom

Tom,
I never tried this, but always wondered about using a 1/4 wave section of
triaxial transmission line, Should be able to get the length right with an
MFJ 259/269. Another idea would be a piece of 1/2 inch ID copper braid.

Tam/WB2TT