On Fri, 22 Jul 2011 23:13:25 +0000 (UTC), Owen Duffy
wrote:

You are treading down a well worn path.

Yeah, I know the feeling when I'm stuck in a rut.

There is a flaw in thinking that the shield of the coax below the bottom
of your nominal dipole does not carry significant current on the outer
surface. Whilst you are trying to account for the current distribution
on the nominal dipole, you are ignoring the common mode current on the
feedline.

Agreed. The feedline should be decoupled, which one can do by
wrapping a few turns into a coil and tied together with tie wraps.
Another method is a 1/4 wave decoupling sleeve further down the coax
cable.

The antenna will 'work' and you may be delightedm but if you expect that
it will have very low VSWR based on formula dimensions, I think you will
be dissapointed.

Well, putting a 75 ohm antenna onto a 50 ohm system results in a
minimum VSWR of 1.5:1 assuming the coax is cut for some multiple of
1/2 wave electrical length. Also, most HT's are designed to tolerate
the fairly miserable VSWR presented by rubber ducky antennas and body
proximity.

In this case, the problem to be solved was rather mundane. A friend
was in the process of packing for a weekend camping trip with the
kids. His 13 year old daughter apparently had lost the rubber ducky
antenna from her Yaesu FT-60r. Rather than borrow an antenna, or
build an adapter kludge, he decides it's time for her to build an
antenna. They found some SMA cables, and proceeded to make a simple
coaxial sleeve dipole. However, there was some debate over the cut
length of the driven element because of the presence of coax
dielectric. I get a hasty email with the question and discover that I
really don't know the answer. So, I posted the question here. We'll
find out how it worked when they return.

Whether the home made antenna is optimized for best performance
doesn't seem to be important in this case. Any reasonable antenna is
better than a rubber ducky, so she should be fine (assuming she
doesn't lose another antenna).

You might obtain better feedline decoupling by some form of common mode
choke.

Yes, that will help (as previously noted).

The failure of the sleeve to effectively decouple the dipole is the
reason why several antennas don't work as thought. King gives some hints
that the relative diameters of the conductors bears on operation.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558