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

[Larry]

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 at 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<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?)


_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk at contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk