helicity on vertical dipoles
 

Hi,

I'm considering a vertical dipole array, one each in 3 corners of a
triangle around the outside of my apartment wall. I'm on the lower
floor, of course. Each vertical dipole will be disguised as a
stereotypical wooden birdhouse that you find at the craft store,
supported by wooden poles with wire cores.

The reason I am considering the vertical dipole is so that I do not
have to bother with a radial system, I would rather hide the grounding
system inside the pole support.

Only disadvantage of the scheme is that my roof is only 25 feet high,
and that the bottom of a vertical dipole should be 10 feet off the
ground. Under normal circumstances a 40 meter vertical dipole at
one quarter wavelength on the side wold be impossible. 30 would be
extremely difficult, 20 just might be doable.

What I would like to try to do is shorten the elements of each dipole
using helical winding rather than straight length wire. That way I
can have the active elements closer to the roof or even a little
above.

Question: to what extent can I shorten the winding while still
retaining good signal apeture? about 50 %? More? And what would I
need to do (if anything) to compensate for this?

Thanks in advance,

The Eternal Squire