On Fri, 21 Jul 2006 23:04:54 +0100, "Reg Edwards"
wrote:


"Walter Maxwell" wrote
The graphs reporting BLE's measurements are pretty conclusive. In
addition, the
BLE paper is the basis on which the FCC set the requirements for the
ground
systems on all AM BC stations since 1939

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

Walt, where've you been lately?

I have no doubt that BLE measurements are good and valid at LF and
below. But to extrapolate conclusions up to HF, where amateurs reside,
and where funny things happen to radials, is somewhat dangerous.

I understand BLE forgot to measure ground resistivity and permittivity
of the site. Perhaps because they thought it didn't matter very much.
But such things certainly matter above about 3.5 MHz. At HF radials
behave very differently from behaviour at LF if only because the
ground 'constants' have changed from their DC and LF values (which are
the values usually inserted in HF computer programs.)
----
Reg.

Hi Reg,

I'll admit to being away from rraa for quite a while. A good bit of the time
away was while finishing the writings for Reflections 3, which includes several
new chapters, some of which archive a portion of my escapades in designing
antennas for various spacecraft, including those that flew on the World's first
weather satellite, TIROS 1. I was fortunate in being at the right place at the
right time when the space age began. Those were the years I spent with Jess
Epstein, the 'E' of the BLE team. Some of the additional material that went into
Reflections 3 is available on my web page at www.w2du.com. I think you might
find Chapters 19A and 21A of interest.

From eavesdropping on the banter between you and Richard C it's easy to see that
your winery has kept your mental physique it top shape. I hope that your
physical physique continues at least in its present condition.

Getting now to BLE, I agree with you concerning the changes in ground
characteristics at HF compared with MF and LF. I don't know if you have a copy
of BLE, but you should know that the BLE experiments were performed at 3 MHz.
Please let me know if you have BLE, because I'd like to email you a copy if you
don't. It will demonstrate the hundreds of measurements taken meticulously to
arrive at the conclusions reported in their paper of 1937.

Of the many results of various combinations of radial lengths and numbers of
radials, the one that that stands out in my mind is the combination of the
longest radial, 0.412 lambda, with the maximum number or radials, 113. This
combination achieved near-perfect ground, yielding a field strength of 192 mv/m,
as compared to the theoretical maximum of 196 mv/m, achieved with perfect
ground. The reference for these numbers is 1000 watts delivered to the antenna
and measured at 1 mile. Notice that the difference between the ideal and actual
field strengths is only 2 percent.

Reg, concerning the difference in ground characteristics with frequencies above
3.5 MHz, please consider this. When the radials are long enough, and enough of
them spaced sufficiently close, the effect is that of nearly perfect ground,
regardless of the actual ground characteristics beneath the radials. Let's
consider a comparison. First, few radials widely spaced. Displacement currents
reach the ground everywhere surrounding the vertical radiator. Currents entering
the ground between the radials diffract toward the nearest radial of higher
conductivity. During its travel toward the radial it naturally encounters the
resistance of the ground. However, with many radials more closely spaced,
currents now entering the ground have a shorter resistance path in reaching the
nearest radial, approaching a negligible value.

My point is that when there is a sufficient number of radials of sufficient
length to approach a nearly-perfect ground, the ground characteristics beneath
the radials are irrelevant within the area they cover in determining the
terminal impedance and efficiency of the radiator. Therefore, the different gr
ound characteristics that prevail as the frequency increases above 3.5 MHz are
also irrelevant. This is not to say that the ground characteristics away from
the immediate area are not important.

You might get a chuckle concerning the number of radials being 113. The original
plan was to plow in 100 radials. When the grunts Jess Epstein and Bob Lewis had
plowed in the intended 100 there was wire left over on the spool. They asked
Brown what they should do with the remaining wire, he said, "Plow it in." The
remainder of the wire allowed just 13 more radials to be plowed in.

On a personal note, I engineered and built WCEN in Mt. Pleasant, Michigan, 500 w
on 1150 Kc (it was 'Kc' then), with a 300' Blau-Knox tower. I plowed in 120
radials. My Dad manufactured the plow, which I rode, while he drove the tractor.

Hope this keeps ya busy fer a while,

Walt, W2DU