[TowerTalk] Short Ground Rods??

[Red]

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