At 10:56 PM 3/5/98 -0500, Larry Higgins wrote:
>For some time, I've been trying to create some mental image with which to
>visualize the induction into ground which takes place within the induction
>field around an elevated radial. ,,,
>Also, as we penetrate into the
>soil I find it very difficult to imagine much current flowing very far
>below the surface. My only basis for this statement is pure intuition.
>And I have no idea whatsoever what the geometry of the current might be.
>Comments anyone?
>73
>Larry, N9DX
Hi Larry, et al,
I would imagine the same skin effect is responsible for the depth of radial
current flow in soil as it is for current in the antenna near field and
fresnel zone, i.e., the lower the frequency, the deeper the skin effect
"penetrates". I've been giving a great deal of thought to this whole
problem, as my long-envisioned mountain top tower is getting closer to
fruition. I have poorly conducting soil (deciduous forest and very rocky
soil), typical of the Appalachian highlands. My present antenna is a 1/4
wave Inv. L, with about 50 ft vertical, the feedpoint elevated about 10
feet, with four elevated radials, roughly NE,SE,SW,NW, all trimmed to the
same length. There are also about a dozen on-ground radials from 1/8 to 1/3
L in the same directions as the elevated radials. The antenna works fairly
well. The thought was to reduce the ground loss between the elevated system
and ground by concentrating ground radials directly underneath the elevated
radials. (It also allows radials to follow more or less level ground; the
north-south directions are steeply up and downhill) This is in keeping with
the article by ON4UN awhile back, stating that even one elevated radial is
sufficient, two can cancel some of the horizontal radiation, etc., and I
experimented and ended up with four, relatively balanced orthogonally, to
cancel some of their horizontal radiation. Current losses between the
elevated radials and lossy ground are, hopefully, helped by the on-ground
system.
As far as the new system being built, it seems some experimentation is in
order. The big question is whether to stick with vertical radiators, with
their potential ground losses or try to gain enough height and terrain slope
to make horizontal radiators effective. I go back and forth with this, but
all the theoretical background indicates that, no matter what I do (short of
copper plating the near/fresnel zones) I get 3-6 db loss from my poorly
conducting soil/rock. Horizontal signals do not suffer to this degree, so
by using sloping terrain to "increase" effective antenna height, they may
win out, even on 160. We'll see...
Interesting thread, glad to see the comments here.
73,
John, K4IQ
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