> How high do you think the lower end of a vertical dipole
for 75 meters
> should be to be reasonably efficient? 8-10 feet? Or
would I still need a
> ground screen?
Modeling programs have the largest error when used in
situations like this (an actual measurement by Haagn and
Barker of low dipoles show losses were significantly higher
than NEC-2 predicts when a dipole was somewhere well below
.05 wavelength height).
We can be sure NEC-2 models are best-case for efficiency,
and NEC-2 shows a ground screen produces considerable
efficiency improvements in 80m dipoles lower than 35 feet.
In A-B tests of low dipoles (which are not as accurate as a
FS meter test, but they still mean something) I saw 5dB and
My best guess would be at 1/2 wl not much is gained or
lost, but below 1/4wl you'd probably better have a screen or
grid of wires parallel with the antenna if you are concerned
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
TowerTalk mailing list