[TowerTalk] Grounding with Polyphaser lightning arrestors

[jimlux]

Larry wrote:
> I believe it was the Polyphaser book that suggested that ground rods should 
> be spaced no more than twice the length of the ground rod. So, for common 8 
> foot ground rods the spacing should be not more than 16 feet apart. There 
> was a rationale to the spacing but it escapes me at the moment -  charge 
> distribution during a strike most likely.
> 

more like that if you put them closer, the effectiveness (measured as DC 
resistance) is less.  Two rods close together have a DC resistance 
(referred to infinitely far away) more than half the resistance of one 
rod.  The limiting case would be if they were driven touching each 
other, which would have a resistance roughly the same as one rod.  Get 
them two rod lengths apart, and the resistance is "close" to half that 
of one rod.

Why not put them hundreds of feet apart then?
In lightning protection, though, you also want to minimize the length of 
the wires to the rods (because of the R and L of the wire). And, wire is 
expensive.

So, about two rod lengths apart is a good compromise.