[TowerTalk] [Bulk] Re: RF Ground is a Myth

[Jim Brown]

On Wed,1/21/2015 1:01 PM, Patrick Greenlee wrote:
> I'm confused...    (still, yet, again...)
>
> There are recommendations to place ground rods (8 footers) 8 ft apart 
> in a circle (octagon.) If my 14 ft apart Ufer grounds are too close 
> together so as to act as a single point, what about the 8 ft rods 8 ft 
> apart? 

Forget about "single point" with respect to these various earth 
electrodes. Multiple electrodes (rods, Ufer, etc.) simply provides 
multiple paths in parallel to discharge a strike. Each of those paths is 
R + jwL, and the impedances to earth of the combination of those paths 
adds like any other combination of resistance and inductance, except to 
the extent that mutual coupling between the electrodes increases the 
combined impedance.

Lightning is NOT a DC event, it is an RF event, with the energy 
concentrated in very broad spectrum, roughly centered (on a log 
frequency scale) around 1 MHz (in other words, spread roughly between 50 
kHz and 20 MHz). Thus, the inductance dominates the impedance. 
Resistance matters primarily to the extent that we would like the 
conductor to carry as much as possible of the strike current before it 
vaporizes. :)

73, Jim K9YC