"Bob Bob" wrote in message
...
Hi Y'all! (said with a strange Australian accent!)

Am doing so NEC work for my QTH and need to get an idea of the ground
conditions here.

I started with the "Rich soil" of DallasLincoln but was advised that it
was more clay and likely to be typical of that in central VA.

Any information helpful.

Cheers Bob W5/VK2YQA



The 4NEC2 manual extract is attached inline below;


Pastoral, low hills, rich soil, typical from
Dallas, TX, to Lincoln, NE 0.0303 20 Very Good

Pastoral, low hills, rich soil, typical of OH
and IL 0.01 14 Good

Flat country, marshy, densely wooded, typical
of LA near the Mississippi River 0.0075 12

Pastoral, medium hills, and forestation, typical
of MD, PA, NY (exclusive of mountains and
coastline) 0.006 13

Pastoral, medium hills, and forestation, heavy
clay soils, typical of central VA 0.005 13 Average

Rocky soil, steep hills, typically mountainous 0.002 12-14 Poor

Sandy, dry, flat, coastal 0.002 10

Bob, According to my 1977 edition of "Reference Data for Radio Engineers"
Soil conductivity along the south bank of the red river is 30mS/m (Most of
the OK side is shown as 15 mS/m). Since the map is not very detailed, and
without going to the extent of graphic overlays, it seems that Dallas is in
a region of lower conductivity at 15 mS/m. If you are interested I can scan
the map for you.

I went to the trouble of measuring my soil conductivity; using the "4 rod
method", with 60 Hz AC, as per the ARRL handbook (Measured 52 mS/m in
Calgary). I believe I have JPEGs of the relevant pages someplace. Also
some guys I was working with, a couple of years ago, devised a method of
measuring the complex permittivity with a capacitor structure. There were
some problems with the method, which they eventually corrected. The
information was required to analyze short range VHF transmission
underground.

73.

Frank, VE6CB

I went to the trouble of measuring my soil conductivity; using the "4 rod
method", with 60 Hz AC, as per the ARRL handbook (Measured 52 mS/m in
Calgary). I believe I have JPEGs of the relevant pages someplace. Also
some guys I was working with, a couple of years ago, devised a method of
measuring the complex permittivity with a capacitor structure. There were
some problems with the method, which they eventually corrected. The


Frank,

The measurement method in the Handbook is seriously flawed. You will
almost always measure something many times better than the soil really
is at radio frequencies.

Since soil conductivity varies widely over small distances, and since
it also has seasonal variations, a rough guess from a book is about as
good as anything.

Myself, I don't worry about it. I just use average soil in models.

73 Tom

The measurement method in the Handbook is seriously flawed. You will
almost always measure something many times better than the soil really
is at radio frequencies.

Since soil conductivity varies widely over small distances, and since
it also has seasonal variations, a rough guess from a book is about as
good as anything.

Myself, I don't worry about it. I just use average soil in models.

73 Tom

Thanks for your comments Tom, and you raise some valid points. Jerry Sevick
"The Short Vertical Antenna and Ground Radial", pp 25, 26, does state that
the procedure is accurate to within 25%, but does not provide any
independant verification of these claims. The method was developed by M. C.
Waltz at Bell Labs, but, again, nothing was ever published.

It would be interesting to develop a more accurate method. While my
measurement of 52 mS/m may not be very realistic it is evident that this
region does have a very high soil conductivity. Ground-wave daylight
reception of AM broadcast stations, with strong signals, at well over 300
miles is possible.

73,

Frank

Frank's wrote:

Thanks for your comments Tom, and you raise some valid points. Jerry Sevick
"The Short Vertical Antenna and Ground Radial", pp 25, 26, does state that
the procedure is accurate to within 25%, but does not provide any
independant verification of these claims. The method was developed by M. C.
Waltz at Bell Labs, but, again, nothing was ever published.

It would be interesting to develop a more accurate method. While my
measurement of 52 mS/m may not be very realistic it is evident that this
region does have a very high soil conductivity. Ground-wave daylight
reception of AM broadcast stations, with strong signals, at well over 300
miles is possible.

There are more accurate methods to calculate ground conductivity, but
what's the point? The skin depth in soil is on the order of 10 or 20
feet, depending on the frequency and soil quality. This means that
substantial current is flowing down to a few times this depth. Certainly
where I live, and I'd bet that in most locations, the conductivity is
far from uniform. So in order to know the conductivity of the soil which
is carrying current, you'd need to measure it down to several tens of feet.

Once you had that data, what would you do with it? Currently available
modeling programs assume homogeneous ground to an infinite depth. So
you'd have to choose some single value from among your measurements if
your objective is to get better accuracy from a program. But there's no
evidence that a homogeneous ground with any single value of conductivity
will behave the same as a stratified ground.

So having even an extremely accurate measure of surface conductivity at
a particular radio frequency (and it does vary with frequency) still
gives you much too little information to build even a crudely accurate
model of the actual ground in which the current is flowing.

Roy Lewallen, W7EL

Hi Roy..

And I only wanted a simple answer. sigh Was just trying to limit some
of the variables and learn some at the same time!

The FCC map says its roughly 8 mS/M here... That will do..

Cheers Bob VK2YQA

Roy Lewallen wrote:

There are more accurate methods to calculate ground conductivity, but
what's the point? The skin depth in soil is on the order of 10 or 20
feet, depending on the frequency and soil quality. This means that

There are more accurate methods to calculate ground conductivity, but
what's the point? The skin depth in soil is on the order of 10 or 20 feet,
depending on the frequency and soil quality. This means that substantial
current is flowing down to a few times this depth. Certainly where I live,
and I'd bet that in most locations, the conductivity is far from uniform.
So in order to know the conductivity of the soil which is carrying
current, you'd need to measure it down to several tens of feet.

Once you had that data, what would you do with it? Currently available
modeling programs assume homogeneous ground to an infinite depth. So you'd
have to choose some single value from among your measurements if your
objective is to get better accuracy from a program. But there's no
evidence that a homogeneous ground with any single value of conductivity
will behave the same as a stratified ground.

So having even an extremely accurate measure of surface conductivity at a
particular radio frequency (and it does vary with frequency) still gives
you much too little information to build even a crudely accurate model of
the actual ground in which the current is flowing.

Roy Lewallen, W7EL

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.

Frank

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.

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