>From: k6se@juno.com (Earl W Cunningham)
To: <topband@contesting.com>
>Date: Sat, 14 Mar 1998 14:24:19 EST
>
>On Sat, 14 Mar 1998 10:59:04 -0700 Eric Gustafson Courtesy Account
><n7cl@sparx.mmsi.com> writes:
>
>>(snip)
>
>"The correctly sized, sufficiently dense screen is superior to four
>resonant radials in close proximity to earth."
>
>>(snip)
>
>Please define "correctly sized" and "close proximity to earth" as used in
>this context.
>
>
>Tnx, 73, de Earl, K6SE
>
Sure Earl,
I thought I had done so in the original post. But here is what I
had in mind when I wrote that.
1. Correctly sized
Large enough to extend to the edge of or just beyond the near
field zone around the radiator. For a full sized (or nearly
so) 1/4 wave radiator this is a radius of approximately 1/4
wavelength. Significantly shorter radiators can get by with
smaller screens.
But there is danger in applying (2*(D^2))/W. It is itself a
rule of thumb. However, for full sized linear radiators it
works pretty well as long as the image element is included in
D.
2. Close proximity to earth
Close enough so that the near field zone under the plane of
the radials is in contact with the earth's surface. With six
or fewer radials, this is somewhere between 1/4 and 3/8
wavelength. But the loss is not terribly significant until
the region between 1/8 and 1/4 wavelength is violated. Below
1/8 wavelength, the losses are considerable. For amateur
purposes, with four radials, less than 1/4 wave would be a
reasonable definition to use.
This height will decrease as the density of the radial screen
is increased. So the more radials that are used, the lower
the base can be without incurring significant loss. This
holds up to a radial density that constrains the distance
between the ends of the radials to 0.015 wavelengths. At
which point the radial plane can be coincident with the
earth's surface without incurring significant loss _from this
source_ (there will still be earth interaction loss in the
far field zone beyond the screen limits).
73, Eric N7CL
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