Steve:
Polyphaser's The Grounds for Lightning and EMP Protection, Second
Edition, has this to say about radial grounds on rocky soil (p.22):
In this situation [rocky soil], numerous slightly shorter lengths
[radials], along with using solid flat strap radial conductors, has been
proven to work. The copper strap's sharp edge will concentrate the E fields
that are present due to the existing L di/dt voltage drop and breakdown or
arc onto the surface of the rock or soil.
And:
On solid bare rock, straps will help spread out the charge on to the
surface of the rock.
What I take away from these passages is that, on rocky soil, one should
use a multitude of copper straps that are not too long (< 75 feet) and that
cover a wide area. (And keep your fingers crossed.)
73 de
Gene Smar AD3F
----- Original Message -----
From: "Steve London" <n2ic@arrl.net>
To: <towertalk@contesting.com>
Sent: Wednesday, August 03, 2005 4:41 PM
Subject: Re: [TowerTalk] Sparky Pays a Visit
> So, just for amusement, let's see what folks would do in my situation...
>
> I live on a very dry hilltop in SW New Mexico. There are large limestone
> boulders at the surface, or within a few inches of the surface. You don't
dig in
> this stuff with a shovel or backhoe - it takes a jackhammer or excavator
with a
> jackhammer attachment. There is no moisture in the ground, except for a
few days
> after a monsoon rain event.
>
> For what small good it might do, I have ground rods at each end of my
beverages.
> The deepest I could get them was about 2 feet. The resistance between
the
> ground rods (which are about 500 feet apart) is around 5M ohms. We're
talking
> DRY here !
>
> Frankly, what the local power company does for grounding seems worthless.
They
> drill a hole in the ground for their pole, run their copper ground wire to
the
> bottom of the pole, loop the copper wire in a spiral around the bottom of
the
> pole, and plop the pole in the ground. With no moisture in the ground, I
can't
> believe that does anything. When my house was built, they didn't even try
to
> drive in a ground rod, they just used the rebar and concrete in the
foundation
> as a Ufer ground. I understand that is okay for safety grounding, but
> completely worthless for lightning protection.
>
> Now, as far as radio goes, I have a 105' guyed tower, about 100 feet from
the
> house, and two 30' guyed towers about 30 and 50 feet from the house. The
tall
> tower has the typical Rohn-approved concrete base and guy anchors. No
ground
> rods, buried copper wires, etc. All of the cables, from all of the towers
run
> on top of the ground, and terminate at an aluminum plate ("bulkhead")
located
> about 30' from the house. There is a separate set of cables that can be
> connected to the bulkhead connectors, running underground, through plastic
> conduit, to the ground-floor hamshack. When the hamshack cables are not
> connected to the bulkhead, they sit in a covered plastic bucket, about 3
feet
> from the bulkhead. The bulkhead is not intentionally grounded or connected
to
> the house Ufer ground. My strategy is simple - I don't care what happens
on the
> tower, but keep the lightning out of the house.
>
> I haven't had a direct hit yet, but that's only a matter of time on this
hill.
> It does make me feel like I'm playing with a loaded gun !
>
> So, short of doing a major excavation to lay a circle of copper tubing
around
> the house and bonding everything to it, what might make sense to protect
the house ?
>
> 73,
> Steve London, N2IC/5
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|