I have a friend who is a ham and is EE with 30+ years of antenna design
experience. He tells me emphatically, "concrete is not a good conductor."
Given this, I'd be trying to not depend on concrete as part of a grounding
system. Rebar installed correctly in steel reinforced concrete is probably
at least 2 inches under the surface of the concrete on any side, top, or
bottom. Weld plates installed when the concrete was wet (to give you
something to anchor to) or bolts or other components set into the concrete
are typically NOT in contact with the rebar cage inside the concrete (which
is not a good conductor anyway.) So again, it is better to not rely on the
concrete to conduct.
73 Patrick AF5CK
-----Original Message-----
From: Jim Lux
Sent: Wednesday, December 26, 2012 8:58 PM
To: towertalk
Subject: [TowerTalk] Fwd: Re: Earthing a tower
-------- Original Message --------
Subject: Re: [TowerTalk] Earthing a tower
Date: Wed, 26 Dec 2012 17:28:43 -0800
From: Jim Lux <jimlux@earthlink.net>
To: Jim <zl2bmh@xtra.co.nz>
On 12/26/12 3:14 PM, Jim wrote:
Hello.
I hope I have sent this correctly, as it's my first post.
Is it necessary to earth a tower?
Yes..
If your tower is securely bolted to a concrete base with bolts and
there's a decent amount of rebar in the base, it's probably pretty well
grounded/earthed.
The concern is whether the bolts are in close contact with the concrete
with no tiny crack for water to get in and corrode the steel, or for the
water to create steam in the event of a lightning stroke. Typically,
this is assured by keeping the top of the concrete above grade and
sloped so that water doesn't just sit on it.
If it's just sitting on the bare soil, you might want to do some more
grounding.
A bigger issue is whether the coax shields are bonded at the bottom of
the tower, or where they come into your house, etc.
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