As I recall having the rods too close was less effective in its ability to
dissipate the strike charge. It wasn't a pure resistance idea although that
is also a consideration.
It has been awhile since I read about the distance and, alas, my memory
isn't what it used to be.
73, Larry W6NWS
----- Original Message -----
From: "jimlux" <jimlux@earthlink.net>
To: "Larry" <w6nws@arrl.net>
Cc: <towertalk@contesting.com>
Sent: Tuesday, October 07, 2008 7:53 AM
Subject: Re: [TowerTalk] Grounding with Polyphaser lightning arrestors
> 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.
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