Richard Harrison wrote:
Roy, W7EL wrote:
"What conclusions could we draw from that information?"

My comment is a little off topic as it is not about measuring soil
constants. It is only an opinion that the FCC`s decisions regarding a
standard grounding system for medium wave broadcast stations worked out
very well. 120 redials each about 1/4-wavelength seems to work well
whether soil is good or bad.

In summer or winter, if the ground cracks open from drought or is
covered with a foot or more of floodwater, the tower currents and field
strengths hardly change at all. Directional patterns are unaffected.
Amazing and well done!

Well, not exactly amazing, since the FCC deliberately requires the
ground to be covered by so many radials that the location and its ground
conditions don't matter any more.

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.

The real technical question is: how many, and how long, will be "just
enough" for "here"? That obviously requires a lot more knowledge and
engineering judgement.

Having just taken delivery of two miles of radial wire, the question of
"How much is enough?" is starting to become very practical...


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Ian White GM3SEK wrote:

. . .
The real technical question is: how many, and how long, will be "just
enough" for "here"? That obviously requires a lot more knowledge and
engineering judgement.
. . .

And for the purpose at hand. We have to keep in mind that the
requirements for AM broadcasters are quite different from those of
amateurs. A few percent difference in field strength means a few percent
difference in a broadcaster's audience size and therefore in advertising
income. This income difference is felt year after year, so any change
that brings a few percent increase in field strength is worth a fair
amount of money for a broadcaster to implement. On the other hand, a
difference of 1 dB (more than a 20% change in efficiency or 10% change
in field strength) is seldom worthwhile at all for most amateurs.

I'm not sure why the great hangup on how many radials AM broadcasters
use. It certainly isn't what most amateurs need.

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
...
Ian White GM3SEK wrote:

. . .
The real technical question is: how many, and how long, will be "just enough"
for "here"? That obviously requires a lot more knowledge and engineering
judgement.
. . .
Well, Ian, the BLE paper reports data allowing one to make that engineering
judgement. It's unfortunate that my copy of the paper is in my library in
Florida, and I won't be back there until November to scan it for the group.
However, I have ordered a copy from the Michigan State U library.

The BLE experiments were conducted to determine what combination of radials
would form the best simulation of a perfect ground, i.e., what combination would
achieve a field strength closest to the ideal calculated value. One factor they
considered is that when the spacing between adjacent wires in a grid structure
is 1/20 lambda or less, the effect is that of a continuous reflecting surface.
The spacing between radials is not exactly the same as a grid structure, but the
effect is similar.

BLE found that the optimum length of the radials in the ground is not related to
resonant length as it is with elevated radials. They found that the principal
reason for the optimum length concerns the volume containing the significant
energy in the electromagnetic fields in the space surrounding the radiator that
intersects the ground. They found that at a distance of 0.4 lambda from the
radiator the energy in the fields has reduced to the level of diminishing
returns, where collecting the currents at a greater distance would yield no
significant decrease in loss resistance, and therefore no further increase in
field strength. Indeed, the field strength obtained with at least 90 radials 0.4
lambda in length was found to be insignificantly less than that of a perfect
ground. This fact was unknown prior to BLE's experiments. I can't remember the
exact difference shown in the graph, but it is inconsequential.

With the radials simulating a near-perfect reflecting ground plane the skin
depth of the earth beneath the radials is of no consequence, because the RF
energy is nearly totally reflected, with only an insignificant amount
transmitted through the ground plane. Consequently, the soil conditions directly
beneath the ground plane are irrevelant.

However, the soil conditions immediately external to the ground plane are
important to the intensity of the ground wave propagation from vertical
radiators. The poorer the soil conductivity the greater the loss at low angles
of elevation. And as we all know, propagation of the ground wave is frequency
sensitive. Many years ago, using the FCC propagation charts of field strength vs
distance for a conductivity of 8, the geographical area covered with a field
strenght of 1 mv/meter at 1 mile for a 250 watt station at 550 KHz would require
47 kilowatts at 1500 KHz to cover the same area with the same signal level.

When I receive the requested copy of the BLE paper I'll scan it and publish it
for all to see.

Walt, W2DU

"Walter Maxwell" wrote in message
...

"Roy Lewallen" wrote in message
...
Ian White GM3SEK wrote:

. . .
The real technical question is: how many, and how long, will be "just
enough" for "here"? That obviously requires a lot more knowledge and
engineering judgement.
. . .
Well, Ian, the BLE paper reports data allowing one to make that engineering
judgement. It's unfortunate that my copy of the paper is in my library in
Florida, and I won't be back there until November to scan it for the group.
However, I have ordered a copy from the Michigan State U library.

The BLE experiments were conducted to determine what combination of radials
would form the best simulation of a perfect ground, i.e., what combination
would achieve a field strength closest to the ideal calculated value. One
factor they considered is that when the spacing between adjacent wires in a
grid structure is 1/20 lambda or less, the effect is that of a continuous
reflecting surface. The spacing between radials is not exactly the same as a
grid structure, but the effect is similar.

BLE found that the optimum length of the radials in the ground is not related
to resonant length as it is with elevated radials. They found that the
principal reason for the optimum length concerns the volume containing the
significant energy in the electromagnetic fields in the space surrounding the
radiator that intersects the ground. They found that at a distance of 0.4
lambda from the radiator the energy in the fields has reduced to the level of
diminishing returns, where collecting the currents at a greater distance would
yield no significant decrease in loss resistance, and therefore no further
increase in field strength. Indeed, the field strength obtained with at least
90 radials 0.4 lambda in length was found to be insignificantly less than that
of a perfect ground. This fact was unknown prior to BLE's experiments. I can't
remember the exact difference shown in the graph, but it is inconsequential.

With the radials simulating a near-perfect reflecting ground plane the skin
depth of the earth beneath the radials is of no consequence, because the RF
energy is nearly totally reflected, with only an insignificant amount
transmitted through the ground plane. Consequently, the soil conditions
directly beneath the ground plane are irrevelant.

However, the soil conditions immediately external to the ground plane are
important to the intensity of the ground wave propagation from vertical
radiators. The poorer the soil conductivity the greater the loss at low angles
of elevation. And as we all know, propagation of the ground wave is frequency
sensitive. Many years ago, using the FCC propagation charts of field strength
vs distance for a conductivity of 8, the geographical area covered with a
field strenght of 1 mv/meter at 1 mile for a 250 watt station at 550 KHz would
require 47 kilowatts at 1500 KHz to cover the same area with the same signal
level.

When I receive the requested copy of the BLE paper I'll scan it and publish it
for all to see.

Walt, W2DU

In my previous post above I forgot to mention that the displacement currents
that enter the ground between the radials don't follow the lossy ground to the
center of the radial system. Instead, they quickly diffract to the nearest
radial and thus continue toward the center along the radial wire. Consequently,
the more radials the shorter distance the diffracted current has to travel to
reach the higher conductivity of the wire. The last I knew the FCC requires only
90 radials (every 4°) to comply with the regulations, but many BC antenna
engineers use 120 (every 3°).

I discussed this issue in Chapter 5 in both the 1st and 2nd editions of
Reflections, with a diagram of the diffraction phenomenon in Fig. 1.

Walt, W2DU

Walter Maxwell wrote:

In my previous post above I forgot to mention that the displacement currents
that enter the ground between the radials don't follow the lossy ground to the
center of the radial system. Instead, they quickly diffract to the nearest
radial and thus continue toward the center along the radial wire. Consequently,
the more radials the shorter distance the diffracted current has to travel to
reach the higher conductivity of the wire. The last I knew the FCC requires only
90 radials (every 4°) to comply with the regulations, but many BC antenna
engineers use 120 (every 3°).

I discussed this issue in Chapter 5 in both the 1st and 2nd editions of
Reflections, with a diagram of the diffraction phenomenon in Fig. 1.

This interaction among radials has quite a dramatic effect on the
effective ground conductivity. I noticed and reported quite some time
ago that Reg's ground radial program produced answers which disagree
strongly with both BLE and NEC-4 modeling (which agree with each other
reasonably well), and speculated that he didn't account for this
interaction in his program. (I haven't checked since to see if the
program has been modified.) All he says about having to trust the writer
of the program if you don't have access to the source code is true.

Roy Lewallen, W7EL

Walter Maxwell wrote:
. . .

When I receive the requested copy of the BLE paper I'll scan it and publish it
for all to see.

Is the publishing of copyrighted papers on the Web generally permitted
under fair use rules? The IEEE and other publishers of professional
papers charge around $25 for downloaded reprints, and I'd think that
would cut into their income. Or does the IEEE specifically permit
publishing of their papers on the Web?

Roy Lewallen, W7EL

On Wed, 22 Jun 2005 08:57:00 -0700, Roy Lewallen
wrote:

Walter Maxwell wrote:
. . .

When I receive the requested copy of the BLE paper I'll scan it and publish it
for all to see.

Is the publishing of copyrighted papers on the Web generally permitted
under fair use rules? The IEEE and other publishers of professional
papers charge around $25 for downloaded reprints, and I'd think that
would cut into their income. Or does the IEEE specifically permit
publishing of their papers on the Web?

Unless I'm mistaken, the copyright would have had to been renewed to
remain in effect.

According to this link:

http://www.scils.rutgers.edu/~lesk/copyrenew.html

it was not.

On Wed, 22 Jun 2005 09:33:33 -0700, Wes Stewart
wrote:

According to this link:

http://www.scils.rutgers.edu/~lesk/copyrenew.html

it was not.

Hi Wes,

You should take care to observe the proviso offered:
"This file does not contain listings for music, movies, or
periodicals."

The practice of many journals is that your material, offered for
publication, is accepted only with the explicit rights of ownership
being transferred to that society. In fact, if you were to cite your
own work without giving a reference to that society's publication,
then you could be held accountable for plagiarism.

The "Open Source" movement has sparked a debate in this regard and
academic authors are being better versed on methods that allow them to
both publish and retain rights.

73's
Richard Clark, KB7QHC

Wes Stewart wrote:

Unless I'm mistaken, the copyright would have had to been renewed to
remain in effect.

According to this link:

http://www.scils.rutgers.edu/~lesk/copyrenew.html

it was not.

Thanks for the URL - it looks like it'll come in handy. Unfortunately,
the fourth sentence on that page is "This file does not contain listings
for music, movies, or periodicals." The BLE paper was published in the
_Proceedings of the IRE_, a periodical.

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
...
Walter Maxwell wrote:
. . .

When I receive the requested copy of the BLE paper I'll scan it and publish
it for all to see.

Is the publishing of copyrighted papers on the Web generally permitted under
fair use rules? The IEEE and other publishers of professional papers charge
around $25 for downloaded reprints, and I'd think that would cut into their
income. Or does the IEEE specifically permit publishing of their papers on the
Web?

Roy Lewallen, W7EL

Hasn't the copyright expired on material published in 1937?

Walt