In a message dated 96-09-17 10:09:30 EDT, you write:
>
>Anyone have any experience using or modeling a shunt fed vertical
>with elevated radials? Without isolation at the base of the
>vertical, it seems doubtful the impedance of the elevated radials
>would be low enough to help much, but I'm not sure.
>--
>Scott K9MA
Hi Scott, This question has been "bugging" me for years now. I have
attempted to measure a system here, and I have measurements made at a
broadcast station (made by other people) showing elevated radials, at least
in these two cases, do not live up to claims. They are about 4 dB or more off
from predictuions.
There is also data that shows NEC fails to model a low dipole accurately so
far as ground loss is concerened, in that case a low dipole modeled by NEC-2
shows about 4 or 5 dB more signal in the model than the real world.
My Beverages also measure more current taper than NEC models, indicating
losses would be higher than predicted.
I'd like to see more real A-B field strength tests, hang the impedances!
I also have an open question about radiation. If anyone knows the answer, I'd
appreciate a direct E-mail.
If I have a wire one hundred feet long and ten feet off the ground, and it
carries one ampere of current at the feedpoint and over a thousand volts at
the open end, how do I keep that wire from coupling to the ground by electric
and magnetic fields, as well as radiation fields?
The only way I know of to cancel those effects are by having a wire with
equal and opposite distribution right next to the original wire (in that case
it's a transmission line instead of a radial). The only way I know of to
reduce that effect is by spreading the current out over more wires, reducing
the nearfield electric field and magnetic fields and radiation fields from
the wire by reducing the voltage and current in the wire.
What magic is at work in elevated radials that allows the fields to just stop
below the wires, and why wouldn't it show up in direct measurements of fields
near the ground below and between the wires?
73 Tom
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