> Even Sevik was fooled by this. He thought we could measure earth
resistance
> at low frequencies (power line frequencies) and extrapolate that data to
HF
> conductivity. The method even made it into Handbooks. As far as I know it
> has been retracted now.
>
> One fellow on 160 meters was heartbroken because he estimated only a few
> mS/m conductivity using this method, while another was elated because he
> measured over 50mS/m using this method. In fact, neither data meant
anything
> at all because you can't measure at 60Hz and tell anything at all about
what
> the soil behaves like at 6MHz.
George Hagn (formerly at SRI) did a lot of work on measuring actual ground
parameters at RF, using a open two wire transmission line. He actually
started using dipoles (for which idealized theoretical models are available)
and comparing ideal vs measured.
There's also work from a variety of researchers in modeling EM
characteristics of soil, layered or not, homogenous or not. J.R.Wait is
probably the best author to start with.
>
> The reason for this is actually at the root of the problem in using any
> model to predict performance of horizontal wires (like radials) close to
> earth. The earth is generally far from a uniform media with constant
> characteristics. All models do is treat the earth like a big "soup" with
the
> same uniform characteristics, and that is almost never the case. The model
> works OK for a reasonably high wire because such a wide cross section is
> illuminated the average characteristics are what matters. It does not work
> nearly as well for a wire close to real earth.
>
> So you have two problems. One is knowing what the soil acts like in all
> these different layers. the second is applying that data to a model that
> allows you to model the soil exactly as it appears.
>
> I don't know of any modeling program that allows you to enter soil
> conditions in multiple layers, and I don't know of any simple method of
> measuring those layers (which also vary in every direction) at radio
> frequencies.
There are a variety of codes that do layered earth, some with
discontinuities. Some work was done with modeling far field performance of
HF antennas using NEC-BSC in a sort of 2D fashion, with the terrain modeled
as a series of rectangular chunks, each with its own dielectric properties.
None of these codes are available in a "ham ready" form, or, for that
matter, in a "ready to go" form. All of the codes that conceivably would do
a decent job modeling (assuming you had sufficiently detailed ground
properties: no easy matter in itself) are, shall we say, computationally
intensive. You'd need a fairly burly high performance computing facility to
do anything useful (but then, people DO have access to 128 processor
clusters and such, so it's not out of the question). Of some interest is
that the determination of the ground properties is usually done by taking a
set of small scale measurements over the area, and numerically inverting the
measurement data with some sort of iterative scheme to determine the
estimated model, which can then be used for a larger scale simulation.
If you look at transactions and conference proceedings for IEEE A&PS and
ACES, you'll turn up a fair amount of stuff.
>
> Most people can't measure coil Q or ESR, and so they don't have that data
> either. Most of the coil Q modeling programs are far off from measured
data.
>
Here is an area where there ARE very accurate modeling programs available
(sub percent accuracies, given mechanical dimensions and material data).
INCA is one (and free, to boot). The switching power supply business has
fostered a lot of research into good analytical or numerical methods for
measuring such things.
Jim, W6RMK
_______________________________________________
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