I agree with WOØW's comments below, and will add a few more. In the Rocky
(aptly named) Mountains, we have a tough time as well. I couldn't install
my tower below ground, so I drilled 25 holes in the rock, and cemented the
re-bar in place, then built the base around it. I had the same problem
with ground rods. I went with a "horizontal" installation, following the
advice of local microwave tower installers, as well as incorporating
guidance from Polyphaser and I.C.E. I used a 1 inch copper tube to
surround the base of the tower, then used 1.5 inch copper straps on the
surface to run out along the rocky ground. May not be "perfect", but its a
start.
http://home.centurytel.net/WD0M/
Click on Ham radio, then choose from the Tower Installation, Lightning
Protection, or others....good luck with your project!
73,
Joe
WDØM
At 09:23 AM 5/19/2005, Red wrote:
>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.
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