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

 To: Re: [TowerTalk] Thoughts on a Grounding System for my new tower "Larry" Larry Mon, 3 Sep 2012 11:41:05 -0400 mailto:towertalk@contesting.com>
 The radial and rods were underground. I will try to dig out the book and reread it. It has been some time (10 years?) since I last read it. There may have been more to the rationale than I remember. 73, Larry W6NWS -----Original Message----- From: Jim Lux Sent: Monday, September 03, 2012 9:18 AM To: towertalk@contesting.com Subject: Re: [TowerTalk] Thoughts on a Grounding System for my new tower 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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