Re: NEC-LIST: ground conductivity of grassy soil

The difference between a perfectly conducting ground plane in the
"vicinity" of the antenna and one of finite loss is not large so long as
the majority of the displacement currents in the high current "near
field" area are efficiently returned by the ground radial system. And
the necessary extent of that system is dependent upon frequency, and to
a much lesser degree on the burial depth and local soil conductivity.
George Brown's measurements were conducted at 3 MHz and not properly
analyzed for skin effect, and so his prescription of 90 to 120 radials
~90 degrees in length is fairly conservative and quite so for the lower
end of the MF band, as we recently documented in some work on military
Psyop antennas. I gave a paper on this at the IEEE Fall BTS Symposium a
year or two ago, co-authored by my partner Steve Lockwood, who made most
of the measurements. (Not my only criticism of Brown, but that's
another story.) Local "near field" conductivity (so long as a reasonable
ground system is employed) is not nearly as important as "vicinity"
conductivity in determination of final signal strength at modest
distances from an MF antenna.

Ben Dawson

Benj. F. Dawson III, P.E.
Hatfield & Dawson Consulting Engineers, LLC
9500 Greenwood Avenue North
Seattle, WA 98103 USA
206 783 9151
206 789 9834 Facsimile

GrantBingeman_at_cs.com wrote:
> Here is another interesting question that just cropped up: can the ground
> conductivity at standard AM broadcast band frequencies be improved near a tower
> if dry, barren earth is enhanced with a crop of live, green grass that is kept
> moist with an agricultural sprinkler system? g
>
> Grant W. Bingeman, P.E.
>
>

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