Andrew, Just enter in EZNEC :

Conductivity = 0.1 milli-Siemens,

( Or Resistivity = 10,000 ohm-metres. )

Permittivity, or dielectric constant K = 4,

and you will get results just as accurate as EZNEC is able
to provide with more usual values of ground 'constants'.

At HF, in the case of granite, as discussed below, the
controlling factor is permittivity K and any reasonable
very high value for resistivity will suffice. Granite is a
fairly low-loss material. Roy, you should have more
confidence in your 'baby'.

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Soil, or whatever you are blessed with, has an
impedance Zs = Rs - jXs. It can be modelled to the
first order of accuracy by a resistance in parallel
with a capacitance.

Rs and jXs are the equivalent series components.

Soil resistivity is that measured between opposite
faces of a 1-metre cube of the material. Easy to
visualise and understand.

Capacitance is that measured between a pair of
1-metre square plates spaced 1-metre apart,
multiplied by the permittivity of the material.

Unless iron filings or crushed ferrite are mingled with the soil, the
permeabilty of this 1-metre cube of space will be unity and can be ignored.
Such mixtures are not known in the natural world.

Soil loss occurs due to current flowing in the soil
through Rs. Note carefully that Rs, a series
component, reduces as frequency increases. And
so soil loss decreases with increasing frequency.

Permittivity K increases fast with water content. In
solid rock moisture content is negligible and K is
that of the material itself, crudely 3 to 7. Mica is
a rock, K = 7. Quartz, another rock, K = 4.
Sahara sand has a lot of dry air in it, K = 3.

Lumping all sorts of backyards together, soil impedance is resistive below 1
or 2 MHz with a small -ve angle. Around 5 to 12 MHz impedance has an angle
around 45 degrees. At 30 MHz impedance is that of a lossy capacitor, but
low in value and therefore a good reflector. At VHF soil impedance is very
low, has a high negative angle, and is an efficient low-loss reflector.

The best reflectors are those with a small impedance magnitude relative to
377 ohms combined with an impedance angle approaching -90 degrees. The
angle of incidence is also in there somewhere. But don't forget skin depth
in the soil is quite shallow at VHF and only the top thin surface layer may
be involved.

If anybody intends to do any calculations it's far more convenient and
meaningful to work in practical terms of ohm-metres. Milli-Siemens sounds
pseudo-scientific and is not particularly impressive.

The capacitance between two 1-metre square plates, spaced 1-metre apart, is
8.8 picofarads. Doesn't seem very much. But with an ordinary garden soil
permittivity of 15, the impedance at 7 MHz is - j170 ohms. And with an
ordinary garden soil resistivity of 170 ohms-metres we have an angle of -45
degrees.

Two programs, EARTHRES and RADIOETH may be of interest. Download in a few
seconds from the following website and run immediately. In addition,
program SOILSKIN calculates skin depth in soil.

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Regards from Reg, G4FGQ
For Free Radio Design Software
go to http://www.g4fgq.com
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