On Thu, 2 Sep 2004 15:10:33 -0500, (Richard
Harrison) wrote:
Coastal Texas is almost as good as it gets when it comes to soil on the
map, 30 millimhos per meter. Seawater is not shown on the map but its
conductivity is given as 5,000 millimhos per meter or 167 times as good
as the best soil.

Hi Richard,

If all would review the standard FCC groundwave propagation curves,
they would notice that they offer low AM Band signal strengths in
terms of "conductivity" and that the differences in strength for the
5,000 millimhos per meter and that of 40 millimhos per meter (125 fold
difference) DO NOT achieve the same proportional difference in
received signal strength. In fact, the difference is so narrow you
could shave with a razor as sharp as it. Even at the high end of the
band the difference has to be out 700 miles to show the "conductive"
ratio. Of course, over that range of transmission ONLY Sea Water
would support that forecast as continental soil varies vastly in
smaller spans - hence the reputation of the Sea.

John Cunningham says in "The Complete Broadcast Antenna Handbook: on
page 309 that:
"The conductivity of the earth ranges from about 2 millimhos per meter
for dry sandy locations to as high as 5 mhos/m for sea water."

This is the conundrum of conductivity of earth: that there is so much
contradiction. You cite coastal Texas, but in distinct contradiction
to the dry sandy locations forecasts, pan handle Texas also exhibits
just as high conductivities for less water as Corpus Christi which
oddly enough easily has twice the sand content as those arid
wastelands.

There are a lot of reductionist statements about ground that simply
don't exhibit against the claims made for it. Reggie has long claimed
to be an authority on the subject, and when push comes to shove for my
hints that he offer Kelvinian substance, he complains about the CIA or
his insufficient understanding of English. The urchins of Rio would
guffaw at that one. I will anticipate Punchinello's Magic 8-Ball
answer "try again later," as I am sure the software could do no
better. ;-)

And by the way, the kitchen calibration of mud is a common gardener's
exercise, why Punchinello cannot or does not recite it is evidence
that no one really cares to ask (me or him). His embarrassment would
be found in its lack of correlation to RF (there are far more
variables to consider).

73's
Richard Clark, KB7QHC

"Richard Clark" wrote
If all would review the standard FCC groundwave propagation curves,
they would notice that they offer low AM Band signal strengths in
terms of "conductivity" and that the differences in strength for the
5,000 millimhos per meter and that of 40 millimhos per meter (125 fold
difference) DO NOT achieve the same proportional difference in
received signal strength. In fact, the difference is so narrow you
could shave with a razor as sharp as it. Even at the high end of the
band the difference has to be out 700 miles to show the "conductive"
ratio. Of course, over that range of transmission ONLY Sea Water
would support that forecast as continental soil varies vastly in
smaller spans - hence the reputation of the Sea.
_______________

For a reality check, here are the approx distances to the 1 mV/m contour for
1kW of radiated power from a 90 degree vertical with a good radial ground
system. The values were determined from the FCC's standard curves.

Freq Conductivity/Miles

540 kHz 8/66, 40/124, 5,000/140
1,600 kHz 8/22, 40/56, 5,000/126

The average ground conductivity in the U.S. is fairly low, probably
somewhere between 8 and 16 mS/m. The difference in ground wave propagation
over such paths is dramatically poorer than over sea water.

It is also clear from the above values how much better the low freq MW
broadcast channels perform.

RF

Richard Clark wrote:
On Thu, 2 Sep 2004 14:27:37 +0000 (UTC), "Reg Edwards"
wrote:


Incidentally, ground loss is not only smaller in sea water, it is also
smaller with soil resistivities of several thousand ohms and greater.
There's a maximum somewhere in between.


Hi George,

The statement above falls into the category of "Old Wives' Tales."
Given the choice for conductors, Sea Water ranks 6 or 7 orders of
magnitude in worse conductivity than any metal (or even carbon) you
would care to pick. By this logic, you should do everything in your
power to operate in an open pit coal mine. ;-)
. . .

It's not an "old wives' tale" at all.

We have to remember that there are two loss mechanisms involved with
antennas. One is the resistance encountered by ground current flowing to
one of the feedline terminals in a ground-mounted antenna. This can be
reduced to an arbitrarily small value by installing radials. The other
is loss incurred when the field strikes the ground and reflects. This
second loss is generally negligible for horizontally polarized antennas
except at high radiation angles, but is very significant for verticals
at low angles. This loss occurs mainly beyond the far edge of most
radial fields, so there's usually nothing you can do to reduce the
reflection loss except to move the antenna.

Perfectly conducting ground has no loss, and perfectly insulating ground
has no loss. When considering ground system loss (resistance encountered
by local ground currents), the loss is maximum somewhere in between.
Doing some experimental modeling with EZNEC/4 (NEC-4 based), I've found
that the maximum loss for a radial ground system in the HF range
unfortunately hits somewhere around average soil conductivity. It's not
an abrupt maximum -- the loss varies fairly gently with conductivity.

Reflection loss has to be considered a little differently. While the
same statement about perfectly conducting and perfectly insulating
ground is still true, if you had perfectly insulating ground, radiated
power would be lost to useful radiation by penetrating the ground,
whether or not it got dissipated as heat. (In reality, it would of
course eventually get dissipated, since no ground is perfectly
insulating.) Looking just at the amount of radiation that ends up above
the horizon, and neglecting conductive ground current loss, the
reflection "loss" does indeed seem to increase monotonically as the
ground conductivity decreases.

The ground reflection loss can easily be evaluated with any version of
EZNEC, including the demo. Choose a vertical antenna such as example
file Vert1.ez, set the ground type to Real/MININEC, wire loss to zero,
and the plot type to 3D. There should be no resistive loads in the
model. Then look at the Average Gain shown at the bottom of the main
window after doing a plot calculation. The deviation from a value of 1
(0 dB) represents the fraction of the applied power that's lost in the
ground reflection.

Roy Lewallen, W7EL

Maximum ground loss during wave reflection, between low resistance sea water
and very high soil resistivities, could very well be when soil impedance is
of the order of 377 ohms. Or is related to that number.

This is because, on the average of incidence angles, soil is more likely to
absorb than reflect wave energy received from free-space. There's a better
impedance match with free space at 377 ohms. The reflection coefficient is
smaller.

Soil permittivity and frequency will also have an effect on the ballpark in
which maximum absorption and hence maximum transmission loss lies.
---
Reg, G4FGQ

On Sun, 5 Sep 2004 03:04:23 +0000 (UTC), "Reg Edwards"
wrote:

when soil impedance is
of the order of 377 ohms.

Hi Reggie,

Is THIS the soil in your garden? Must be miserable for DX! That
would be an exceedingly deep pile of very dry sand you are on. Quite
remarkable stuff where ever you might find it. Like the green cheese
the moon was supposedly made of, you have more lore from the aged
domestic goddess file than one could have imagined.

What coefficients would that be for your program? What would the
conductance be? How many radials would it resolve to? Can you even
guess the Q? So many questions and easily suitable for the Magic
8-ball "Try again later."

73's
Richard Clark, KB7QHC

**** Post for FREE via your newsreader at post.usenet.com ****

"Reg Edwards"
Maximum ground loss during wave reflection, ...
... could very well be when soil impedance is
of the order of 377 ohms. ...
...impedance match with free space at 377 ohms....

He he he.

What if your garden is in the 'near field' ? (Obviously no "what if" at
all - it almost certainly is.) Then the garden should be some other value,
NOT 377 ohms, to match your locally generated EM wave.

Also, the ground properties would obviously be NON RECIPROCAL (!!!) since
the other guy's signal would presumably be in the far field and would arrive
in the V/I ratio of 377 ohms.




-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
*** Usenet.com - The #1 Usenet Newsgroup Service on The Planet! ***
http://www.usenet.com
Unlimited Download - 19 Seperate Servers - 90,000 groups - Uncensored
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=