> To help you understand this better, do a search on the ARRL website for
> the many excellent technical pieces on vertical antennas and ground
> systems. There is a five-part series on ground systems! Also, study
> some of the things written by W2FMI, Jerry Sevick, about his
> experiments with ground systems for short loaded antennas.
>
> Jim Brown K9YC
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has been retracted now.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.
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.
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.
73 Tom
_______________________________________________
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Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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