"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