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

 To: towertalk@contesting.com Re: [TowerTalk] Thoughts on a Grounding System for my new tower Jim Lux Mon, 03 Sep 2012 06:18:37 -0700 mailto:towertalk@contesting.com>
 ```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
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