Frank wrote:

Certainly, all valid points. I was more interested in actually doing
precise measurements, but considered it might improve my model accuracy. I
even thought of digging a hole to see how the soil varied. Doubt I would
have dug down 20 or 30 ft. Most of the ground here is clay, and then
probably bedrock, at this elevation of just over 4,000 ft ASL. Ansoft's
HFSS, or CST, could probably handle an accurate, stratified, ground model.

Do a web search for "OWL" (qualifying it with ground
conductivity-related terms to cut down the references to the bird and
other contexts). I believe it stands for "open wire line", and the last
I heard, was the standard way of measuring RF ground conductivity. It
involves a buried open wire line, but that's about all I know about it.

There's certainly nothing wrong with learning to measure ground
characteristics as an educational process. No matter what seemingly
useless learning exercise I undertake, I ultimately learn many other
things from it. By all means, go for it.

I'd love to see some results from one of the good field solving programs
for stratified grounds, even something contrived, and even a simple
vertical with buried or elevated ground system. What I'd like to know is
whether there really is a single value you could assign to a single
homogeneous ground and get the same results. I suspect not, but have no
proof one way or the other.

Roy Lewallen, W7EL

There's no need to ridicule measurements of soil resistivity just
because at a deeper layer there is a strata of different resistivity.

Any information is far better than complete ignorance.

Roy, you are just displaying your knowledge of geology.

Obviously, in practice it is the resistivity of the top layer which
predominates anyway.
----
Reg.

Reg Edwards wrote:
There's no need to ridicule measurements of soil resistivity just
because at a deeper layer there is a strata of different resistivity.

Any information is far better than complete ignorance.

Roy, you are just displaying your knowledge of geology.

Obviously, in practice it is the resistivity of the top layer which
predominates anyway.


The top layer here Reg is about one to six inches thick of rich
pastorial soil. Below that is a layer of red clay with high iron
content than can be a few feet or dozens of feet deep, and mixed or
below that are various rocks. At places the rock is at the surface.

Measurements of a ground rod at various places on my farm show anything
from 50 ohms to 500 ohms for a four foot rod measured against a
reference antenna.

What value should I use Reg that would be better than a guess? In the
areas where soil is very dry on top but has wet soil below rock, should
I use the rock or the soil below it?

The fact is the method using multiple ground rods produces numbers that
might have agreed with soil characteristics at the test site, but they
produce some very wild numbers other places. I've seen that method
produce conductivities of over 40mS/m where ground wave attenuation
measurements have shown effective conductivity to be 10-15mS/m.

One fellow on 160 is particularly proud he has 45mS/m soil while his
friend 100 miles away has 5mS/m. The only problem is no one can tell
any difference in their signals, and there isn't any soil in that area
that is over 20mS/m in the AM BC band.

I tghink the best way to estimate conductivity is to measure impedance
of a dipole at low heights, and adjust the modeling program until
impedances match. That certainly seems more reasonable than using 60 Hz
AC on a short ground rod.

73 Tom

Do a web search for "OWL" (qualifying it with ground conductivity-related
terms to cut down the references to the bird and other contexts). I
believe it stands for "open wire line", and the last I heard, was the
standard way of measuring RF ground conductivity. It involves a buried
open wire line, but that's about all I know about it.

There's certainly nothing wrong with learning to measure ground
characteristics as an educational process. No matter what seemingly
useless learning exercise I undertake, I ultimately learn many other
things from it. By all means, go for it.

I'd love to see some results from one of the good field solving programs
for stratified grounds, even something contrived, and even a simple
vertical with buried or elevated ground system. What I'd like to know is
whether there really is a single value you could assign to a single
homogeneous ground and get the same results. I suspect not, but have no
proof one way or the other.

Roy Lewallen, W7EL

Do a web search for "OWL" (qualifying it with ground conductivity-related
terms to cut down the references to the bird and other contexts). I
believe it stands for "open wire line", and the last I heard, was the
standard way of measuring RF ground conductivity. It involves a buried
open wire line, but that's about all I know about it.

There's certainly nothing wrong with learning to measure ground
characteristics as an educational process. No matter what seemingly
useless learning exercise I undertake, I ultimately learn many other
things from it. By all means, go for it.

I'd love to see some results from one of the good field solving programs
for stratified grounds, even something contrived, and even a simple
vertical with buried or elevated ground system. What I'd like to know is
whether there really is a single value you could assign to a single
homogeneous ground and get the same results. I suspect not, but have no
proof one way or the other.

Roy Lewallen, W7EL

The acronym "OWL", could also mean "Optimal Well Locator", which uses soil
conductivity parameters. Dan Richardson's link to a paper by Rudy Severns:
http://www.antennasbyn6lf.com/files/...urements_2.pdf,
does refer to the "Open Wire Line" method of measuring ground conductivity,
and looks extremely interesting.

I have some minimal experience with HFSS (Also known as "Highly Frustrating
Structure Simulator"), and could probably borrow the key to run it at home.
Just the same, to model stratified ground you have to know what is
underground, so you are back to large back yard excavations. I did notice,
during the construction of this house, that the ground appeard to be 100%
clay. I could not see any sign of bedrock.

Frank