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


(VE8AE Andrew) wrote in message om...
I want to build a nice small antenna for my boat. I had a J pole in a
PVC tube, but I thought about a vertical dipole of the sleeve type.
Can you make such a beast with the coax and just remove outer
insulation and then pull braid back over the coax until they are both
the same length? I want it for 156.8 MHz so it should be around 1.5 ft
per side. But I have seen an article that says the top portion should
be smaller than the lower braid portion to balance things out
electrically. Can anyone shed some light on this for me? I have tried
it and my analyzer doesn't show it resonant anywhere near that freq.
Hmmmm.
Help please.
Any other good marine antennas that are easy to build would be great
if you have plans or ideas. Thanks folks.
Andrew VE8AE

"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