[TowerTalk] Lightning protection - length of grounding rodsruns from tower?

[Jim Lux]

On 1/29/15 5:51 AM, Larry wrote:
> The Polyphaser book recommended ground rods spaced at 2x the length of
> the ground rods. The purpose was to spread the charge out in the event
> of a hit. So is that theory or explanation now out, disproved, or ...?
>
> It has been a long time since I looked at the book so I may not have
> remembered it correctly.
>

Yes.. rods spaced closer than 2x the length are less effective (the 
limiting case is if you drive the two rods right next to each other.. 
then it's basically one rod).  Once you're 2xlength apart, spreading 
them further doesn't reduce the impedance much.

Off hand, I think that driving rods close to the concrete doesn't do a 
heck of a lot of good: you've already got a good connection to a large 
area of soil.

A good rule of thumb is to imagine the rod as having a "(hemi)sphere of 
influence" with radius equal to the depth.

If you have a big block of concrete that is 8x8x8 feet, it too has a 
"sphere of influence" (or more a "blob") that extends out quite some 
distance. How far?  You'd have to do some modeling to figure it out.

But I'll bet there's very little to gain by driving a rod within 20 feet 
of a Ufer ground

Note that I'm using "sphere of influence" as a visualization concept.. 
there is no hard cutoff.. the current density gradually decreases.