> Has anyone had any experience (good, bad or ugly) with
> helically wound vertical dipoles?
> characteristics or red flags that are peculiar to
> vertical dipoles?
Helically loaded antennas are generally equivalent to something
slightly above base-loaded and worse than center loaded antennas in
radiation characteristics and efficiency. Of course this can vary
depending on the care and construction put into each, but in general
a good center-loaded system will have better efficiency.
The main advantage of helical loading is the ease of antenna
construction. You don't have figure out how to mount a loading coil,
or wind a good HI-Q weatherproof coil.
The main difficulty with vertical "dipole" systems is keeping the
feedline clear of common-mode currents, and keeping the antenna
halves balanced. The big problem is getting the feedline past the
lower antenna half without inducing excessive common-mode currents.
It is a very difficult problem to handle.73, Tom W8JI