[BC] Elevated vs underground systems

[DANA PUOPOLO]

Actually..
The radials work better if they are insulated. If you look at the current
distributuon, most of it is dissipated in the first 20 feet of non-insulated
wire. THis is the reason that ground screens improve things so mush - much
more area to dissipate. Insulated wire dissipates it more gracefully along the
entire length of the wire. This is also the theory as to why a few radials
that are elevated works so well.

At WHDH (now WEEI) in Boston they used a compromise system designed by Earl
Cullum. They elevated the ground system for the first 50 feet, THEN connected
it to a buried radial system. It's a bitch to maintain, but it does seem to
work well. Unfortunately, the ten foot fact of the half wave towers makes them
electrically more like 5/8 wave; as a result there's some cancellation of the
groundwave signal about 50 miles out due to high angle skywave. Where I used
to live (Rehoboth, MA) I could hear it. The station would ever so slowly
selective fade away until Denver came in instead then slowly ade back in until
it was like a local again.

-D

 

------ Original Message ------
Received: Sun, 12 Feb 2006 01:45:11 PM PST
From: "Phil Alexander" <dynotherm at earthlink.net>
To: "Broadcasters' Mailing List" <broadcast at radiolists.net>
Subject: [BC] Elevated vs underground systems

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


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