On 11/18/22 2:10 PM, Jim Brown wrote:
On 11/18/2022 1:28 PM, Wes wrote:
By serendipity, I measured the input impedance of one radial against
all of the others and it's resonant at 1.84 MHz.
About ten years ago, I did the experiment of modeling a pair of
radials cut to a free-space resonance, starting with them as close as
NEC2 would allow it, and raising them in small increments to several
feet. VF with that closest contact was about 0.7. By 3-4 ft, was close
to free space, taking the #14 THHN jacket into account. I also did
some measurements of THHN #14 pairs laying on the ground, but don't
recall the result.
To a first order, the propagation speed slows as sqrt((1+epsilon)/2)
(i.e. it's like being immersed in a dielectric of half air/half epsilon)
The interesting aspect is that for a dipole (or a wire) on the surface,
a large fraction of the power radiates into the soil, the fraction is
epsilon ^(3/2), so with soil with epsilon 15, about 58 times as much
power goes into the soil, as opposed to radiates up in to the air. So
radials work a LOT better on poor rocky soil than on "good wet" soil.
If you want the theory: Rutledge, D. and M. Muha (1982). "Imaging
antenna arrays." IEEE Transactions on Antennas and Propagation 30(4):
535-540.
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