> So, what is your experience in how high is needed before
the loss reaches
> low level?
There are two loss mechanisms with elevated radials.
The first is a "push against" loss. For every ampere
flowing up into the antenna an exactly equal current must
flow into the ground system.
(something about this is at
and other places)
The second loss relates to the antenna's fields moving
charges in the lossy media around the antenna. In the
Bobtail with the current up high, field density is lower
than normal. You have a strong electric field between the
high voltage hanging ends and earth below and around the
With a high impedance feed, current is fairly low at the
feedpoint. This means "push against" or conduction losses
will be low even with a poor ground.
The major loss (other than Fresnel zone losses) would be
from the HV antenna ends near earth.
A small ground screen or a large number of short radials on
or just above the earth below the wire ends (even those not
fed) would do the most to reduce these losses. The wires
simply serve to provide a low resistance path for
displacement currents at the most concentrated points.
If you have four elevated radials, the answer in the case of
a normal Marconi antenna is 1/8th to 1/2 wl above earth.
Eight elevated radials at 1/4 wl are about equal to a full
system on the earth, four would be a little worse.
It's even a good idea with dipoles (inverted Vee dipoles) to
keep HV wire ends away from earth.
By the way, my mobile antenna beats many home-station
vertical antennas. I am happy with it and make many DX
contacts including come one call SSB contacts into Europe
and the Middle East, and it is about 10-20% efficient on 80
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.
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