http://www.karinya.net/g3txq/temp/ground_zones_2.png
Steve,
Do you know if the elevation chart would change much if instead of a
ground-mounted 80m quarter-wave vertical, a half-wave vertical dipole was
used ? The maximum current point would rise a quarter wave above the ground
and the Fresnel geometry would seem to change, albeit slightly in relation
to the horizontal distance.
Paul, W9AC
.
----- Original Message -----
From: "Steve Hunt" <steve@karinya.net>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Friday, January 07, 2011 9:22 AM
Subject: Re: [TenTec] New and Improved Terminology
Folk who have been following the discussion may be interested in this
chart:
http://www.karinya.net/g3txq/temp/ground_zones_2.png
It shows how the optimum take-off angle of an 80m quarter-wave vertical
mounted over Sandy Soil varies as the radius of an excellent ground
screen (Salt water) is gradually extended outwards. It was produced by
using the capability of EZNEC to have two different radial ground media,
as I described earlier.
Please note that this is simply an exercise in determining how far from
the antenna, ground conditions affect the take-off angle; we're letting
EZNEC do the trig that we could otherwise easily do ourselves with some
simple maths.
Notice that as the ground screen grows from zero to about 100ft - the
range in which we might typically install a ground radial system - the
take-off angle barely changes. At the other extreme, above 500ft we are
beyond the Fresnel Zone in which reflections are taking place, and there
are no more improvements to be had. All the "action" takes place between
100ft and 500ft.
Unfortunately, for many of us that's an area that might well be outside
our property boundary. Even if it is not, at that radius the areas are
so great that we would need to put down massive amounts of copper to
have any significant effect on the ground conductivity. Hence Rudy
Severn's comment: "Any practical ground system will not affect the
radiation angle or far-field pattern!"
73,
Steve G3TXQ
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