George Hagn started with measuring the feedpoint impedance of a dipole
suspended above the ground, and then trying to invert that to ground (and
foliage) effects. He said he eventually went to the Open Wire Line kit
approach (SRI markets the kits), which is basically a two wire balanced
transmission line inserted into the soil. You need to do various depths to
get a good measurement.
Then, there are some papers from folks who have tried looking at the
impedance of a balanced line laid on the surface. Apparently, the problem
here is that while the "line laid between two ideal layers" is relatively
tractable both in an analytical and experimental sense, the "run of the mill
insulated balanced line laid on the dirt" (and weeds) is not. (Having tried
some early experiments with the latter, I can heartily agree... ). Small
variations in spacing of the wires from the dirt have a big effect.
----- Original Message -----
From: "Richard (Rick) Karlquist (N6RK)" <email@example.com>
To: "Tom Rauch" <firstname.lastname@example.org>; <email@example.com>; "Jim Lux"
Sent: Sunday, May 23, 2004 4:00 PM
Subject: RE: [TowerTalk] Dayton 2004 Antenna Forum Papers now on the Web!
> One of the least accurate uses of any
> > model is simulating loss with a wire close to earth.
> > An early comparison of NEC-2 models to actual FS
> > measurements of a low dipole someplace under .01WL high (as
> > I recall). Haggn-Barker did the study in Thailand, and VE2CV
> > Belrose did the modeling showed about 5dB error. My Beverage
> > antennas also show significantly more measured current taper
> > with distance compared to a model.
> Gerry Burke (of NEC fame) told me that some people had tried
> to measure ground characteristics by running transmission lines
> at low heights above the ground. I don't know the exact
> details of these tests, but they sounded like they were similar
> to measuring Beverage current taper. As with your beverage,
> the current taper was surprisingly high. What was really interesting
> is that there was no set of numbers that could be plugged into
> NEC that would produce a model in agreement with the measurements.
> What NEC shows for ground loss is reaches a peak value (as
> the parameters are varied) which is less than the measured value.
> Away from this peak value, as you move towards a pure dielectric
> or a pure conductor, the loss predicted is less than the peak, which
> make sense.
> My strategy is to monitor the drive impedance at the base of
> the vertical and stop adding radials when the impedance nears
> its asymptote. In that way, I have correctly modeled my
> own ground, whatever it's characteristics. For my ground,
> 60 radials is about the useful limit. As always, YMMV.
> Rick N6RK
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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