Folks,
I'm going to disagree with Jim, on two points.
1) Early work done by consulting engineers, using elevated
radials (10'), in place of 120 buried radials indicated that
field strength at 1 mile was equal. I suspect that careful
inspection at points BETWEEN the radials would have revealed
lower field strengths due to earth losses, but I never saw
enough data to know.
As a practical matter, I've been able to throw up three or
four radials on any given band, and have it work just fine.
That includes a full sized elevated ground plane for 160, in
Vermont. Radials were just above moose antler height.
Clearly, it would have been better with more radials, and
I never measured the efficiency. What I did was throw it up
in the tree, and work the expedition on campbell island that
night.
The key is...it very much depends on your terrain, and height
above ground. A GP on 40 or 30, elevated .25 wavelength or more,
with sloping radials, is a killer antenna. But you MUST get it
well above earth to keep the losses down.
Jim's comments about losses are correct. Problem is, you don't
know where 'earth' really is. In dry, rocky soil, it might be
30' down, depending on the nature of the rocks.
2) I was managing a station on 1510KHz, where vandals cut and
pulled out the ground radials on all 4 towers. Efficiency went
from the middle 90% vicinity, down to about 15%. That WAS
measured. And it was before we'd thought about elevated radials
for bcdst use, or we might have proposed it as a temporary system.
More than 6dB down.
Jim n2ea
jimjarvis@ieee.org
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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
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