"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