Jeff Liebermann wrote in
:

....
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.

Those all may reduce common mode current. If you have tried them and
measured the results, you know by how much.

....
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.

And the answer is not a simple as put by some. Particularly, if you had
in mind that the sleeve should be an electrical quarter wave to current
flowing on the outside so forming half of a centre fed dipole, and an
electrical quarter wave on the inside to form a quarter wave s/c stub,
the presence of high permittivity dielectric on the inside prevents
those both happening together... so you compromise, like discarding the
centre fed concept, tune the s/c stub for resonance (max decoupling, and
the sleeve will be less than a physical quarter wave), now tune the top
part of the dipole (it will be a bit longer than a physical quarter
wave) for lowest VSWR on the feedline.

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.

To some extent, everything 'works'. Just stripping a quarter wave of
shield off the end of a length of coax probably 'works' about as well.


Whether the home made antenna is optimized for best performance
doesn't seem to be important in this case.

Indeed, it seems that for most modern hams, performance isn't an issue
or of interest.


Owen