When considering grounding systems, do not overlook two major
categories: soil characteristics and characteristics of the energies you
are trying to ground.
Important soil characteristics are conductivity and reactance. It is
useful to think of the soil as a lossy capacitor, especially in the
spectrum occupied by lightning. Even if conductivity is very poor,
capacity will provide a path for dissipating lightning energy. That
path is more effective at the higher frequencies and drops to zero at DC
The energy in lightning is mostly composed of components at frequency of
1 MHz and below. Earth dissipates it by a combination of conduction and
reactive current. Model a system of ground rods and radials as a number
of capacitors paralleled by resistors and interconnected by elements
composed of distributed capacity, inductance, and resistivity. This is
oversimplified, but may help in understanding what happens.
Thus, the capacitance between the ground system and earth contributes a
large part of that dissipation, especially in soil of poor
conductivity. A screen of many radials covering a large area of earth
contributes a large part of the dissipation of lightning.
Some authorities, including some quoted in literature from PolyPhaser,
suggest that placing ground rods horizontally in dry, rock soil is an
acceptable alternative to the commonly recommended practice of placing
them vertically at a space of twice their depth.
Transmission to earth may also be enhanced by increasing the area of
contact between grounding elements, either rod or radial, by using
elements of greater area or by enhancing their area by encasing them in
concrete. The latter is sometimes further enhanced by the use of
concrete of higher than usual conductivity. I don't recall specific
products or manufacturers, but I'll bet other contributors will provide
that information.
Grounding in the HF spectrum, much higher in frequency than the spectrum
of lightning, is quite different.
73 de WOØW
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|