It is fairly easy to measure soil conductivity at DC and power
frequencies. See program EARTHRES. Such measurement values apply up to
a few hundred kHz.

A few years back I became curious about what happens to conductivity
at HF. And at what frequencies does soil permittivity begin to matter.
Most amateur activity is at HF and above. Yet, without even thimking
about it, we persist in plugging in power frequency values into
formulae and computer programs.

A volume of soil between a pair of electrodes behaves as a resistance
in parallel with a capacitor.

So this is what I did -

1. Obtain a 16-inch length of galvanised steel tube, 5 inches in
diameter. (Mine was industrial ventilation duct)

2. Block off one end of the tube with a rigid disk of plastic
insulating material.

3. Obtain a 17-inch length of copper water pipe, diameter = 0.6
inches.

4. Locate the water pipe in the centre of the tube, resting on the
plastic disk at the bottom.

5. You now have a coaxial structure of accurately known dimensions.
When empty, Zo = 128 ohms. 1/4-wave at 191 MHz

6. Obtain a mixed sample of soil from various places in your garden
under your antenna.

7. Fill the galvanised tube in easy stages with garden soil. At each
stage compress and pack-down the soil to about the same density as it
was in your garden. Make sure the soil is in contact with the inside
surface of the tube.

8. Cover the top of the soil in the tube with a flat disk, with a
hole in the middle, to discourage evaporation of moisture and drying
out of the soil.

9. You now have 16-inch length of transmission line on which you can
make HF impedance measurements using instruments as simple as
hand-held antenna analysers. Resistance measurements at 50 or 60 Hz
can be obtained from volts/milliamps. Although connecting leads can be
kept very short it is advisable to correct measurements for
lead-length above 10 MHz. Measurements were made up to VHF.

10. Using classical transmission line formulae in reverse, the values
of line conductance G, capacitance C and hence permittivity K of the
"insulating" material, i.e., the soil, can be calculated.

11. Measurements are of input impedance of the line with the other
end open cicuit. The basic equation is Zin = Zo*Coth(A + jB) where A
is line attenuation and B is line phase shift. At the lower
frequencies the line is very short and G and Capacitance and then K,
can be calculated directly from measurements and line dimsnsions.

12. A clinical thermometer can be inserted deep in the soil. If the
test cylinder is too large to fit in the domestic fridge, by leaving
the test sample out of doors overnight in winter the effects of
temperature can be observed as the sample slowly warms up from
freezing. Soil has a high negative temperature coefficient of
resistance. Resistance increases as temperature decreases. My garden
soil is roughly -2 percent per degree C at 20 degrees C.

13. I have made HF measurements in other shaped containers, usually
smaller and plastic, with copper sheets for electrodes. Also in the
garden itself between radials and arrays of relatively short rods.
Any sort of measurements are more useful than none.

Some people say the only way to deternine soil characteristics is to
construct a 1/4-wave vertical antenna, feed it with 50 Kwatt at 500
KHz and measure field strength at 1 mile intervals for 100 miles. And
then do some calculations. Don't you believe it!
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Reg G4FGQ