My program doesn't do strap, so instead of 2" strap that the OP has, I
substituted 1" wire which gives a slightly smaller surface area... the OP
said he was going have 700' of buried stuff so that is what I used... of
course you can't bury in rock, but I can simulate it as a worst case, the 1'
of topsoil on top shows there isn't much difference.
David Robbins K1TTT
AR-Cluster node: 145.69MHz or telnet://k1ttt.net
From: Michael Goins [mailto:email@example.com]
Sent: Friday, November 16, 2012 15:28
To: Jim Lux
Subject: Re: [TowerTalk] ground rod depth problem due to rocky soil --
Impractical analysis because where are you going to get 1" wires and who
could possibly afford 700 feet of it? Plus the cost of getting it a foot
down in rock?
I'm on pretty much solid rock here, 1800 feet up on the side of a hill, and
that would cost a fortune to do.
Still not clear about the need/reason. Why not just totally disconnect and
reconnect the tower when you operate? That's what I do here and the tower is
not grounded other than by its proximity due to the factory base 5 feet deep
in solid rock. My station is completely broken from the tower when I am not
operating and I don't operate in storms. There is nowhere for lightning to
go that would harm my station.
Michael Goins, k5wmg
University of Texas at San Antonio
On Fri, Nov 16, 2012 at 8:57 AM, Jim Lux <firstname.lastname@example.org> wrote:
> On 11/16/12 6:46 AM, K1TTT wrote:
>> I plugged that into my lightning modeling software... if I put 7
>> radial wires(NO rods) 1" in diameter (a bit smaller surface area than
>> 2" strap), bury it 1' in solid rock (ignoring any advantage of
>> topsoil above the
>> and run them out 100' (to equal your 700') I get a ground resistance
>> of about 8 ohms... as good as 20 rods 8' long by 1" diameter in good
>> still sounds like overkill to me. If I allow for 1' of topsoil on
>> the rock
>> with the wires on the rock it goes down to under 6 ohms. To put that in
>> perspective, the 'normal' 2 rods in loam calculates out to over 70
>> ohms, in common clay that reduces to 35 ohms.
> This brings up an interesting question... Off hand, I would expect
> that laying a bunch of radials (not too long, but lots of them) of not
> very big wire (AWG14 or AWG12) might be a better lightning ground for
> the dollar than any of the other strategies (except concrete and
> wire.. great ground, and concrete is cheaper than copper)
> Time consuming to install (compared to driving a single rod)
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