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!
----
Reg G4FGQ

Hi Reg,

You've presented a very interesting way of measuring soil characteristics. When
I return to Florida in November I'm going to use your method of measuring the
soil underneath the dipole whose impedances I measured over the frequency range
14 to 15 MHz at various heights above ground, including one set of measurements
with the dipole lying on the ground.

One of the reasons I offered to distribute the data from my measurements is to
see whether anyone can deduce any soil characteristics from the changes in
impedance with height. The changes are significant. For example, the terminal
impedance with the dipole on the ground runs from 470 + j250 at 14 MHz to 570 +
j132 at 15 MHz. The inductive reactance doesn't become capacitive until the
dipole is 2 ft off the ground. In addition, except at zero height, the
resistance component decreases with height, but for every height the resistance
increases with frequency. Do you think any of the soil characteristics could be
determined by such data?

Would you like a copy of my data, just fer the helovit?

Walt, W2DU

Well Reg,

After years of harping on about your lack of a method, you rummage
this up:

On Mon, 20 Jun 2005 10:49:54 +0000 (UTC), "Reg Edwards"
wrote:
....
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.

I don't see you asking Walt for his data to CONFIRM your method. I
don't see you doing any where near Walt's effort in building a sample
matrix of your own to test against your method to CONFIRM your method.
Validation seems to be an orphan in this discussion.

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!

Bosh! Some people indeed. Your biology instruction in the British
school system apparently didn't teach you the difference between
people and straw-men.

The veiled suggestion of a result
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.
is representative of an extremely thin veneer, ignoring the bulk that
is so easily found by using the antenna in situ - the method you
dismiss as unbelievable, and what is experienced every day by
absolutely every Amateur on "Earth."

Reg, it was a nicely scripted recipe. It contains well explained
methods. It attends practical issues of measurement. However, it
wholly lacks common sense when you reject what is already observable.
What you offer is minutia of an old wife's tale.

For method, any existing antenna's free-space characteristics is far
better understood and revealed through a model than what you offer.
And that antenna's free space Z characteristics compared against
measured in situ Z performance yield the solution of what contribution
local earth has to offer. Single point measurement of contaminated
soil samples has as little chance of doing the same as trying to
measure the ocean's capacity with a teaspoon.

73's
Richard Clark, KB7QHC

"Walter Maxwell" wrote in message
...
Hi Reg,

You've presented a very interesting way of measuring soil characteristics.
When I return to Florida in November I'm going to use your method of
measuring the soil underneath the dipole whose impedances I measured over
the frequency range 14 to 15 MHz at various heights above ground,
including one set of measurements with the dipole lying on the ground.

One of the reasons I offered to distribute the data from my measurements
is to see whether anyone can deduce any soil characteristics from the
changes in impedance with height. The changes are significant. For
example, the terminal impedance with the dipole on the ground runs from
470 + j250 at 14 MHz to 570 + j132 at 15 MHz. The inductive reactance
doesn't become capacitive until the dipole is 2 ft off the ground. In
addition, except at zero height, the resistance component decreases with
height, but for every height the resistance increases with frequency. Do
you think any of the soil characteristics could be determined by such
data?

Would you like a copy of my data, just fer the helovit?

Walt, W2DU

I was working at a company a few years ago, and they built a capacitor of
two plates, about 6" on a side, and 0.25" separation. They measured the
thing on a network analyzer, and then packed the dielectric with potting
soil. Again measuring the results on the network analyzer they were able to
deduce the conductivity and permittivity of the soil. I did not think
potting soil was typical, but still an interesting experiment.

Frank

PS, Walt, I would very much like to receive your experimental dipole pdf.

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 simpler method that avoids all the digging and repacking of
the soil is described at

http://www.antennasbyn6lf.com/2005/0...arameter_.html

I've been meaning to try it at my qth this summer.

Torsten
N4OGW/5

I received a request from Frank, VE6CB, in the post directly above Richard's,
asking for a copy of my dipole impedance data.

I tried unsuccessfully to send to Frank, but my server rejects his email
address. Can someone help me get a msg to Frank?

Walt, W2DU

Walter Maxwell wrote:
I tried unsuccessfully to send to Frank, but my server rejects his email
address. Can someone help me get a msg to Frank?

Walt, did you remove the "nospam" from Frank's address
before attempting to send? His address appears to be:

--
73, Cecil http://www.qsl.net/w5dxp

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I have a paper showing measurements of soil conductivity made by the
accepted method of using a buried open wire line, which sounds similar
to your method. The authors found that the conductivity varies
considerably with frequency. Have your measurements shown this also?

Roy Lewallen, W7EL

Reg Edwards wrote:
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 -
. . .

"Walter Maxwell" wrote in message
...
I received a request from Frank, VE6CB, in the post directly above
Richard's, asking for a copy of my dipole impedance data.

I tried unsuccessfully to send to Frank, but my server rejects his email
address. Can someone help me get a msg to Frank?

Walt, W2DU

Sorry Walt, but I put in some anti spam characters in my e-mail. My actual
address is

Thanks,

Frank

Although knowledge of surface ground conductivity and dielectric
constant is much better than nothing, unfortunately it's still not
enough to get truly accurate results from modeling or calculation. Most
grounds are stratified, as anyone can discover by digging a few feet
down, with layers which can be vastly different. The skin depth at 3.5
MHz in average soil is 16 feet, and at 7 MHz and above about 13 feet(*),
which means that significant current flows to depths several times these
values. To make an accurate representation of the ground would require
knowledge of the ground characteristics to several tens of feet.

Fortunately, horizontally polarized antennas are quite insensitive to
ground characteristics except at high elevation angles. So for amateur
purposes, it's of interest mostly for vertical antennas and NVIS
operation with horizontal antennas.

To accurately predict the field strength for vertically polarized
antennas, you would need to know ground characteristics typically for a
few hundred feet from the antenna (depending on the height and current
distribution on the antenna and the elevation angle of interest) -- the
distance at which the field from the antenna strikes the ground and
reflects to interfere with the direct field. This is admittedly a
simplification of what really happens, but the distance where the
effects occur is approximately right. You also need to know the ground
characteristics in the close vicinity of the antenna to evaluate the
efficiency of an antenna having a radial system. Besides the problems of
stratified ground, many of us live in an urban environment where buried
utilities, pipes, reinforcing bar, and houses are likely to be within
the radius of interest.

Finally, even if you had exact information about each layer of a
stratified ground, I don't know of any readily available program which
can make use of that information. NEC based programs like EZNEC can
accommodate only a single ground characteristic, and assume that the
ground is homogeneous to an infinite depth. I'm not convinced that
there's any single value which can be substituted for a stratified
ground which will behave like the stratified ground. This is the weakest
point of currently available modeling programs. But even if they could
model stratified ground, you'd have to know values for each ground layer.

The method used in broadcasting, where the skin depth is on the order of
25 feet(*), of measuring the attenuation of a ground wave signal over a
lengthy path is a much more accurate way of determing overall path loss
than simple measurement of surface ground characteristics, since it
automatically takes into account all the factors I've mentioned.

Again, knowledge of surface ground characteristics is better than
nothing, but don't be fooled into thinking that it gives you real
information about the ground in which the RF current flows. Had Brown,
Lewis, and Epstein measured the surface ground conductivity, it wouldn't
have helped us much.

(*) You might notice that the skin depths don't follow the familiar rule
of decreasing as the square root of frequency. That's because the
familiar rule is valid only for good conductors. Average Earth acts like
a good conductor below about 7 MHz, but above that it's more like a
dielectric, where the skin depth becomes independent of frequency.
Consequently it's not less than 12 - 13 feet at any frequency for
average soil.

Roy Lewallen, W7EL