[BC] Elevated vs underground systems

[Phil Alexander]

On 12 Feb 2006 at 7:54, Stanley Adams wrote:

 > What is the theoretical disadvantage in using the elevated over the ground
 > based system?

High maintenance and exposure to damage.
Theoretically, if you had a disk approximately
3/4 lambda in diameter of pure silver and the
thicknesss of aluminum foil or thinner (if that
were possible) covering the ground around the
base of your tower, it would be electrically
the best ground you could possibly have, for
a day or two until wind, weather, animals or
human thieves destroyed it.

The practical compromise is a diameter of 0.5
lambda of #10 copper wires spaced at 3 degree
intervals. This is not the best ground system,
but it approaches one sufficiently. In some
systems having high base voltages, interspersing
short (typ. 50') radials reduces losses an are
essentially equally effective and less expensive
than expanded mesh that was more often used in
the '50's,

Systems having longer radials are not much more
effective than 1/4 wave unless they are spaced
more closely, perhaps 2 deg. or less. In this
case, field intensity is improved with radial
lengths up to about 140 to 150 degrees.

Unless the ground itself is highly conductive,
such as a salt marsh, burying the radials appears
to have no advantage except for protection.

I've never had the chance to try it, but have
always thought that having the interior part
of the ground system of a high power, half wave
system built as a counterpoise with sufficient
room for walking under it would have a substantial
shielding advantage so the tower might be
approached (for reading actual current) with full
safety. But, since we seldom use direct reading
RFA's these days, there is not much point, except
as a curiosity - and an expensive one at that. <g>


Phil Alexander, CSRE, AMD