It's going to be weeks or even a couple months before I have
time to attack a follow-up to the radial study I reported.
To answer your question on 1/2 or 5/8 radials, they are generally
NOT effective with a 1/4 wave radiator. The feedpoint of a 1/4
wave vertical is a low impedance, while the ends of 1/2 wave wires
are high impedance points. Longer radials like 3/4 wave
will have a lower impedance that is more compatible.
There are some antenna specialists who believe that long elevated
radials experience greater ground losses, as they have a greater
length where they interact with the lossy ground below them. I don't
have enough data to confirm that possibility. I have also heard that
extra long BURIED radials (more than 1/2 wave in length) provide an
improvement in radiated efficiency -- I know of at least one ham
with an 80-meter vertical who added several 2-wavelength radials
toward Europe and swears that it made a significant improvement.
A typical approach for a 80/160 installation would be to have
radials of two lengths in the system -- a set of 1/4 wave radials
for each band.
Would you be willing to rerun the vertical antenna model with elevated
radials and make the radials longer? I would be interested in
5/8-wave and 1/2 wave. I have some possibilities of running verticals
for both 80 and 160, and that is why the 1/2 wave question arises.
The 5/8 simply represents the difficulty in measuring radials. Does
it matter as long as I make them at least 1/4-wavelength?
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