Five years ago, in the general Raleigh area, a number of us did exactly as
you propose. We all used as standard a 151 foot (46 meters) dipole of
insulated wire laying on the ground, spreading out for readings in quite
some number of places across the region. We were looking for velocity
factor, but measured resonant frequency, R at resonance, and R with X at 50
kHz above and below resonance. The wire became known as a Dipole-On-Ground,
or DOG.
We were trying to come up with some measured data particular to an
installation site that could be used for modeling antennas to that
particular site. Also we were looking for something to explain what were
obvious inaccurate predictions from NEC x.x models involving low band
vertical antennas requiring a counterpoise.
We did find that across all the measurements, that velocity factors
computed from the readings varied from 45% to 85%. Often simply reorienting
the dipole 90 degrees around a fixed center (in the same back yard) would
produce very large changes. A front yard would be wildly different than the
back yard. A DOG left in place would vary enormously depending on whether
it had rained in the last 24 hours, or was dry.
Also a DOG laid on top of the grass would vary versus the same layout
notched in the grass down to the dirt. We by this discovery realized that
the reading could not be useful unless it was always notched down to the
dirt, not possible with "volunteer" lawns. Or we would have to set a
standard elevation, say three inches and come up with a support stake kind
of tool to keep the DOG at 3 inches uniformly along its length.
But that never made it into practice because we further found that we could
only rarely come up with a NEC ground constant and distance from ground
that would produce the measured results in a modeled DOG of the same length
and height above ground.
We did adapt the findings into a procedure to estimate the length for best
pattern in a BOG, where the only real issue was knowing the actual velocity
factor to scale the BOG.
After getting our trust in NEC shot out from under us in that experiment,
it's very hard to get any of us to be serious about NEC absolute results
when counterpoise and ground is involved on 80 or 160. High radial density
is the only thing that defeats the irregularity with shielding dense
radials provide in the immediate vicinity of the vertical radiator, in
particular as in FCC spec radials for commercial LF/MF AM broadcast
stations.
73, Guy K2AV
On Sun, Apr 26, 2015 at 1:50 PM, Richard (Rick) Karlquist <
richard@karlquist.com> wrote:
> Is there some way you could lay a half wave
> dipole on the ground and measure the resonant
> frequency and radiation resistance vs the
> known free space values, and convert this to
> ground conductance and permittivity? Perhaps
> model it in some version of NEC and play with
> the ground constants until the modeling program
> agreed with the measured impedance and frequency?
>
> Rick N6RK
>
>
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