[TowerTalk] Thoughts on a Grounding System for my new tower

[Jim Lux]

On 9/3/12 5:13 AM, Larry wrote:
> The use of ground rods spaced at 2x the rod length was covered in the
> Polyphaser book on lightning and perhaps elsewhere. As I recall (it has
> been a few years since I read it)  the idea was better dissipation of
> charge during a strike. The wire between the rods doesn't dissipate the
> charge all that well relatively speaking.

Perhaps they're talking "above ground" for that wire?   If the wire's 
buried it will work as well as a rod.



Closer than 2x rod length
> spacing allows the ground charge to swell and reduces the effectiveness
> of the rods in dissipating the charge.

I'm not sure about "swell", but the 2x recommendation is basically a 
simple rule of thumb based on the current distribution from a rod. TO a 
first order, by the time you're 8 feet away, the current is distributed 
over a cylinder 16 feet in diameter, and pretty small.

The actual equation from the IEEE doc for two idealized ground rods of 
length "L", radius "a", spaced "s" apart is:

R = rho/(4 pi L)*(ln(4*L/a)-1) +
     rho/(4 pi s) *(1 - L^2/(3*s^2) + 2*L^4/(5 * s^4) - ...)

The first term is the resistance of the two rods in parallel, neglecting 
the interaction.  The resistance of a single rod:

R = rho/(2 pi L) * (ln (4*L/a) - 1)


You don't actually gain much as you get farther than one rod length 
apart (formulas from IEEE.. there's one for spaced farther than length, 
and one for closer):

rho (ohm*m)	200
L (m)	3	
a (m)	0.01	
s (m)	1	2	3	4	5	6	7
R	64.6	64.6	64.6	64.6	64.6	64.6	64.6
R2 (s>L)		36.9	35.8	35.2	34.8	34.5
R2 (s<L	41.0	38.1	36.7				




  The Polyphaser book suggested
> several ground radials with ground rods along each radial with the rods
> spaced at 2x the rod length.

I wonder if that's for above ground wire? (that is, the ground 
connection is only the rods, and not the wire?)