On 8/1/2012 9:07 AM, Jim Hoge wrote:
> Team Vertical has employed a gull wing style of raised radials that would
> eliminate your concerns. I'm not the expert here.... just mentioning another
> alternative.
That's my implementation, more or less. My feedpoint is about two feet
above the ground, the radials rise quickly from that level, pulled up at
the far end by an anchor in a tree, at a midpoint by a rope in another
tree, and held up closer to the antenna by sections of 3/4-in PVC
conduit that are held erect by 4 ft sections of small diameter rebar
shoved into the earth. At about ten feet from the feedpoint the radials
are about 7-8 ft high, by 20 ft they're 10-12 ft. We all do what we can
based on our real estate and our resources. I have a LOT of trees and
the earth is mostly soft.
On 8/1/2012 5:47 AM, Charlie Young wrote:
> My models don't show the significant effect of close proximity to ground on
> the radial resonant length.
You can get at least a first approximation by modeling a dipole at the
height where you plan to put radials. I did that this spring in
preparation for this elevated radial project. I did a series of
computations, each for a different height, varying the length for each
height until the "dipole" was resonant. What we are seeing is the
change in the velocity of propagation along the antenna caused by
proximity to the earth.
What I learned is that the velocity factor is on the order of 0.7-0.8
for radials more or less laying on the ground, increasing quickly with
even six inches of elevation, and by the time you've reached 4 ft or so
of height the resonant length is pretty close to what it would be if the
antenna was high in the air. What changes with additional height is
mostly the feedpoint impedance. I should put those plotted curves on my
website.
> Consequently, I just build the antennas, adjust the radials to the real world
> length, and don't worry about what the model says.
Guy has several times recommended tuning the lengths of radials by
connecting them in pairs, dipole-style, to an MFJ259 or equivalent. .
73, Jim K9YC
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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