"Richard Clark" wrote in message
news
On Thu, 23 Jun 2005 11:37:34 -0400, "Walter Maxwell"
wrote:

Seems to me, Wes, that our use on the group could be considered criticism,
comment and teaching. So far I haven't received anything from the library, so
we'll see what happens.

Hi Walt,

Do you have a BailPal account that we can chip into?

73's
Richard Clark, KB7QHC

Sorry Richard I don't know what a BailPal is. Or are you yankin my leg? Or on
the other hand are you being a compassionate soul in case I get sued?

In the meantime I've only had a short time to review the Mathcad info, but I'll
have some questions on it for you shortly.

Walt

Reg, G4FGQ wrote:
"How is it done?"

We have to do it within the USA broadcast frequencies with the following
method.

The site of the transmitting antenna is plotted on a very accurate map.

Pick map sites along radiaal lines from the antenna which are accessible
and free from possible reradiation sources (hard to do within a city)
but many sites along a radial will work.

For a single-tower, the nearest measurement site should be at least 5x
the tower height away. For a directional array, the nearest measurement
site should be at least 10x the widest gap between towers in the array.
You need to be far enough away so the antenna system appears to be a
point source.

You need to make a log of the measurements you make, showing the site
distance from the transmitter, measured field strength, time and
conditions which influence the measurement. You need to be able to
duplicate the measurements. You would prefer to make the first set of
measurements with the antenna operating in a nondirectional mode even if
it normally does not operate nondirectionally, so you can determine
efficiency very simply.

The more sites and measurements, the better. 25 measurements along each
radial is often considered enough for a nondirectional antenna. 40 or 50
would be required 9in a directional array, as the number of radial
measurements needed depends on the complexity of antenna system and its
pattern.

After completing measurements along a single radial, they should be
analyzed to determine the effective field at one mile from the antenna,
and the effective ground conductivity.

Fortunately, the FCC publishes charts are made a part of the rules in
Part 73 of the FCC Rules. You have likely seen reproductions in many
textbooks. I have an old copy of all the groundwave field intensity
versus conductivity charts which divide the AM broadcast band into
frequency segments.

These FCC charts contain more information than we can use, but they also
have what we need.

At the top of the chart is a straight line that shows how the signal
would be attenuated over perfectly conducting earth. The field strength
value at one mile is 100 mV / m. At 2 miles, it`s 50 millivolts / m, and
so on. This is as expected as over perfedt earth the signal varies
inversely with distance from the transmitter.

Beliw the straight line on the chart is a family of curves, each
dedicated to a particular soil conductivity. There is a curve for sea
water, 5.000 millisiemens (millimhos) and there is a curve for about as
nonconductive soil as is found (0.5 millisiemens), and there are several
curves in between those extremes.

All of the FCC curves are based on 100 mV / m at 1 mile, but can be
scaled. If your transmitter delivers 500 mV / m at 1 mile, aimply
multiply all points on the curve by 5.

We want to find the conductivity of our earth. It can be different on
every radial parh from the antenna.We find conductivity by plottibg our
measured field intensities on translucent graph paper with grid lines
which match the fcc graph. Then we line them up and place them over a
light source. We can see which of the FCC curves our points most closely
follow. It`s labeled ewith its conductivity.

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote in message
...
Reg, G4FGQ wrote:
"How is it done?"

We have to do it within the USA broadcast frequencies with the following
method.

The site of the transmitting antenna is plotted on a very accurate map.

Pick map sites along radiaal lines from the antenna which are accessible
and free from possible reradiation sources (hard to do within a city)
but many sites along a radial will work.

For a single-tower, the nearest measurement site should be at least 5x
the tower height away. For a directional array, the nearest measurement
site should be at least 10x the widest gap between towers in the array.
You need to be far enough away so the antenna system appears to be a
point source.

You need to make a log of the measurements you make, showing the site
distance from the transmitter, measured field strength, time and
conditions which influence the measurement. You need to be able to
duplicate the measurements. You would prefer to make the first set of
measurements with the antenna operating in a nondirectional mode even if
it normally does not operate nondirectionally, so you can determine
efficiency very simply.

The more sites and measurements, the better. 25 measurements along each
radial is often considered enough for a nondirectional antenna. 40 or 50
would be required 9in a directional array, as the number of radial
measurements needed depends on the complexity of antenna system and its
pattern.

After completing measurements along a single radial, they should be
analyzed to determine the effective field at one mile from the antenna,
and the effective ground conductivity.

Fortunately, the FCC publishes charts are made a part of the rules in
Part 73 of the FCC Rules. You have likely seen reproductions in many
textbooks. I have an old copy of all the groundwave field intensity
versus conductivity charts which divide the AM broadcast band into
frequency segments.

These FCC charts contain more information than we can use, but they also
have what we need.

At the top of the chart is a straight line that shows how the signal
would be attenuated over perfectly conducting earth. The field strength
value at one mile is 100 mV / m. At 2 miles, it`s 50 millivolts / m, and
so on. This is as expected as over perfedt earth the signal varies
inversely with distance from the transmitter.

Beliw the straight line on the chart is a family of curves, each
dedicated to a particular soil conductivity. There is a curve for sea
water, 5.000 millisiemens (millimhos) and there is a curve for about as
nonconductive soil as is found (0.5 millisiemens), and there are several
curves in between those extremes.

All of the FCC curves are based on 100 mV / m at 1 mile, but can be
scaled. If your transmitter delivers 500 mV / m at 1 mile, aimply
multiply all points on the curve by 5.

We want to find the conductivity of our earth. It can be different on
every radial parh from the antenna.We find conductivity by plottibg our
measured field intensities on translucent graph paper with grid lines
which match the fcc graph. Then we line them up and place them over a
light source. We can see which of the FCC curves our points most closely
follow. It`s labeled ewith its conductivity.

Best regards, Richard Harrison, KB5WZI

Hi Richard, you deserve an A+ for your excellent presentation on the use of the
FCC charts of signal level vs distance and conductivity. You've described the
method exactly as I have used it for single tower BC antennas. I still have a
complete set of the charts from 550 KHz to 1600 KHz that I used during the late
1940s, when I was doing AM BC antenna work.

Walt, W2DU

"Ian White GM3SEK" wrote

That was an administrative policy decision rather than a technical
one.
From the technical viewpoint, everybody agrees that 120*0.25wl is
more
than enough to override the local ground conditions under the tower
irrelevant.

====================================

- - - - and since soil resistivity decreases with increasing
frequency, and the impedance due to soil capacitance also decreases
with increasing frequency, everybody agrees that 1/8th wavelength or
less is more than long enough.

And if that isn't enough, the velocity of propagation along buried
wires is considerably slower than the free-space value. It depends on
moisture content and permittivity.

The attenuation due to skin effect and wire inductance along lossy
radial wires is rather high. There's negligible current flowing in
them at distances greater than 1/4-wavelength at their own velocity.
The wires may just as well not be there.

Finally, as the wires spread apart, at appreciable distance
practically all the remaining current flows in the soil because the
cross-sectional area of the soil is far greater than that of the wire.
The longitudinal impedance of the wire is greater than that of the
soil.

The foregoing applies to low and medium resistivity soils. In arid,
sandy, rocky, cactus-growing soils, with resistivities greater than
5,000 or 10,000 ohms-metres, buried wires have low attenuation, they
become resonant and develop standing waves. It is then a good idea to
consider changing from vertical antennas to horizontal dipoles.

The effects can be estimated by calculations on model radial systems.

It may have been noticed that ground loss is least at low and very
high resistivities. So there must be a maximum loss at some
resistivity. I wonder if maximum loss occurs around 377 ohm-metres
after taking the reflecting angle into account?

If B. L & E, made any errors, they made sure they erred on the safe
side regarding numbers.
----
Reg, G4FGQ

"Reg Edwards" wrote in message ...

"Ian White GM3SEK" wrote

That was an administrative policy decision rather than a technical
one.
From the technical viewpoint, everybody agrees that 120*0.25wl is
more
than enough to override the local ground conditions under the tower
irrelevant.

====================================

- - - - and since soil resistivity decreases with increasing
frequency, and the impedance due to soil capacitance also decreases
with increasing frequency, everybody agrees that 1/8th wavelength or
less is more than long enough.

Reg, do you really mean what you said above, 'soil resistivity decreases with increasing frequency'? Are you sure you didn't mean soil conductivity decreases with increasing frequency? In my experience with AM BC antennas I've found that conductivity decreases, not resistivity.

The FCC charts showing signal level vs conductivity and frequency overwhelmingly show conductivity decreasing with frequency. So you ask, what proof is there that the FCC charts are correct? Well, Reg, soil conductivity measurements of thousands of AM antenna systems world wide have proved them correct.

As an example that I posted a few days ago, consider the coverage area from afforded by a single 1/4wl vertical radiating 250 watts at 550 KHz with a signal strength of 1 mv/meter at one mile and a conductivity of 8. If the frequency were raised to 1500 KHz with a 1/4wl vertical at that frequency, the power required to cover the same area is 47 KW.

Does this example indicate a decreasing soil resitivity with increasing frequency or a decreasing soil conductivity?

Walt, W2DU

Walter,

Your ancient charts, which I think I have once seen but don't now have
ready access to, apply to LF. Permittivity was ignored when they were
calculated. The curves were intended to be used as a guide, better
than nothing, rather than the Bible on the subject.

But amateurs are concerned with what happens at HF. There are a lot of
MHz between 16 KHz, 500 KHz and 40 MHz

I think the discrepancy about conductivity vs frequency is due to
simplification of the equivalent circuit of soil which, in its most
simple form, is a resistor in shunt with a capacitor.

As frequency increases the capacitative impedance decreases and drags
the equivalent resistive component down with it. There is a
significant decrease at around 7 MHz. At 30 or 40 MHz the soil has
changed from being mainly resistive at LF to being mainly capacitative
and not nearly so lossy.

The capacitance between a pair of 1 metre square plates, spaced 1
metre apart, is only 8.8 pF. But when muliplied by the permittivity
of damp soil the impedance at 30 MHz is quite low. The permittivity
of water is 80.

Simple conductivity does not apply. We are not talking about the same
things.

Actually, its not worth arguing about. The uncertainty in soil
characteristics is plus or minus 30 or 40 percent. And it makes less
than 1 S-unit difference to the performance of radials and Eznec take
off angles at HF. No doubt Roy will disagree as a matter of Boston Tea
Party principles. And Richard, KB7QHC, will spin off at a tangent into
Shakesperian verse.

Hope this clarifies my Altzeimer's thoughts on the matter.
----
Reg, G4FGQ

On Sat, 25 Jun 2005 05:42:31 +0000 (UTC), "Reg Edwards"
wrote:

Richard, KB7QHC, will spin off at a tangent into Shakesperian verse.

Hi Reg,

Let's draw a chord between 3 soil samples to see how fruitless knowing
"How to measure soil constants at HF" really is:

You are in farm country where the annual rainfall is 835mm. Where the
mean temperature is 12.8°C. Where the soil is 20% sand, 65% silt, and
15% clay.

What is the Conductivity in the 80M band?

You are in farm country where the annual rainfall is 360mm. Where the
mean temperature is 4.9°C. Where the soil is 65% sand, 20% silt, and
15% clay.

What is the Conductivity in the 80M band?

You are in farm country where the annual rainfall is 790mm. Where the
mean temperature is 6.9°C. Where the soil is 31% sand, 33% silt, and
36% clay.

What is the Conductivity in the 80M band?

OR

We return to our regularly scheduled "More Les Dames d'Escoffier
Recipes with your host Punchinello." Today we discuss packing coaxial
tubes with meringue to measure propagation delay for custards.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Sat, 25 Jun 2005 05:42:31 +0000 (UTC), "Reg Edwards"
wrote:

Richard, KB7QHC, will spin off at a tangent into Shakesperian
verse.

Hi Reg,

Let's draw a chord between 3 soil samples to see how fruitless
knowing
"How to measure soil constants at HF" really is:

You are in farm country where the annual rainfall is 835mm. Where
the
mean temperature is 12.8°C. Where the soil is 20% sand, 65% silt,
and
15% clay.

What is the Conductivity in the 80M band?

You are in farm country where the annual rainfall is 360mm. Where
the
mean temperature is 4.9°C. Where the soil is 65% sand, 20% silt,
and
15% clay.

What is the Conductivity in the 80M band?

You are in farm country where the annual rainfall is 790mm. Where
the
mean temperature is 6.9°C. Where the soil is 31% sand, 33% silt,
and
36% clay.

What is the Conductivity in the 80M band?

OR

We return to our regularly scheduled "More Les Dames d'Escoffier
Recipes with your host Punchinello." Today we discuss packing
coaxial
tubes with meringue to measure propagation delay for custards.

73's
Richard Clark, KB7QHC

====================================

I don't know. And neither do you or anybody else.

If you DID know you would not have the foggiest idea what to do with
the data anyway. I might!

Havn't I recently said the uncertainty in ascertaining soil
characteristics is in the order of 30 to 40 percent and not worth
arguing or making yourself appear ridiculous about.

There's missing data. You forgot the iron oxide content and soil
permeability.
----
Punchinello.

Reg Edwards wrote:
. . .
The attenuation due to skin effect and wire inductance along lossy
radial wires is rather high. There's negligible current flowing in
them at distances greater than 1/4-wavelength at their own velocity.
The wires may just as well not be there.
. . .

I'm afraid your oversimplified model of how radials work has once again
led you astray. B, L, & E's measurements show the following:

For an 88 degree high vertical, where n is the number of radials, the
following fraction of the current at the center is flowing in the radial
1/4 wavelength (at a velocity factor of 0.2, the approximate VF in the
radial's environment), from Fig. 42 of their paper:

n Fraction
15 0.67
30 0.68
60 0.90
113 ~ 1.0

1/4 *free space* wavelength from the center:

n Fraction
15 0.19
30 0.14
60 0.26 [This is a minimum; it rises then drops further out]
113 0.61 " "

Note that the results are quite different when the radiator is only 22
degrees high (Fig. 43) -- the resonant effects apparent on the 60 and
113 radial measurements are absent, and the currents decay
monotonically. There isn't nearly as much difference between 15 and 113
radials. But with 15 radials, the current 1/4 in-ground wavelength from
the center is still about 67% of the current at the center.

Again I see evidence that your analysis overlooks the interaction among
radials. There's less interaction when there are only a few, but even
with 15 your analysis has led you badly astray. And does it account for
the considerable differences with different radiator heights?

But I've pointed this out to you before yet you keep promoting this
myth, so I guess you just don't want to be confused by the facts.

If B. L & E, made any errors, they made sure they erred on the safe
side regarding numbers.

One of their key results is that ". . .the ground system consisting of
only 15 radial wires need not be more than 0.1 [free space] wave length
long, while the system consisting of 113 radials is still effective out
to 0.5 [free space] wave length."

Their results agree reasonably well with NEC-4 modeling. But I'm sure
glad we've got you to set us straight about how well they did and how
they could have improved their methods. You've surely got a clearer
perspective, not having been prejudiced by actually reading their paper.

Oops, here I am nitpicking again -- pointing out that .67 doesn't equal
zero.

Roy Lewallen, W7EL
Certified Reg's Old Wife and Nit-Picker

Reg, G4FGQ wrote:
"Your ancient charts, which I think I have once seen but don`t now have
access to, apply only to LF. Permittivity was ignored when they were
calculated."

True, they are not for HF. My edition was reprinted by the Seabrooke
Printing Company, Inc. and covers the range ftom 540 KHz to 1600 KHz.
Dielectric constant (permittivity) is assumed to be 15 in all cases.

The reason there are graphs for frequency segments such as 1560 kc to
1640 kc is that loss increases with frequency. Skin effect is an
important faxtor. The higher the frequency, the less it penetrates the
earth, so the crust carrying the r-f is thinner. The decline of field
intensity versus distance from the transmitter is steeper at HF.

My set of curves has a page which gives the formulas used to construct
"surface wave field intensity versus numerical distance over plane
earth". It has separate sets of formulas for vertical and horizontal
polarizations. Curves in the book are for vertical polarization, the
only thing of interest to a broadcaster.

One option would be to construct your own set of curves. Another would
be to find some curves which have already been constructed. I don`t know
of any but expect that they exist.

Best regards, Richard Harrison, KB5WZI