Now I wonder: The concrete is conducting due to water content and then we
drive in a few grounding rod and connect them to the tower. Knowing the rod
are
make with a copper layer and the rebar are made of steel, isn't there a big
risk of electrolytic corrosion? I never heard that to be a problem. Is the
high pH the protecting factor?
I know concrete is the insulator between railroad tracks. You use the fact
that the wheels connect the tracks as a mean to check where a train is. I
suspected though that where were some conductivity although I guess it may not
be
considered enough as a good grounding for a lightning strike.
Hans, N2JFS
____________________________________
From: wa2puq@frontiernet.net
To: HansLG@aol.com
Sent: 9/1/2008 9:08:51 A.M. Eastern Daylight Time
Subj: Re: [TowerTalk] Fwd: tower grounding
Well, actually that question was answered by a guy in the 40's name of Ufer.
It was found that the moisture in the concrete made it a very effective
coupler from the bonded re-bar cage, which was also bonded to the tower, to
the
surrounding earth.
IIRC, the story is that it was first used as a method of protecting military
explosive bunkers and then extended to tower grounding by Mr. Ufer who was,
I believe, an FCC engineer.
There were concerns raised by some about the concrete block possibly being
blown apart by the power of the lightning bolt, but that was found to be due
to poor connections within the re-bar cage or to the tower itself causing high
heat and steam at the points of failure. Also concerns were raised about
the concrete surrounding the re-bar rotting it out. But, since the pH of
concrete is high (a base rather than an acidic) corrosion did not occur
(couldn't
use it in buildings and bridges if it did).
I saw evidence of this firsthand some years ago. An 80 foot free standing
commercial tower fell over because of corrosion where it entered the block.
BUT, what actually did it in was the layer of soil (dirt complete with some
wild flowers growing there) which had been allowed to accumulate on TOP of
that
block, the top surface of which was slightly below grade... a word to the
wise out there... hint, hint!
One other consideration. Any ground rods driven to augment the Ufer
grounded tower need to be driven at a distance to the block of TWICE their
length so
as to be far enough away that the strike energy will be low enough in their
vicinity that they will actually add to the dissipative effect. Any closer
and they'll be swamped out by the field at the tower base.
Best way to visualize the earth surrounding the tower is as a big sponge
already filled with water. Splash in some more and the water will initially
mound up at the pour point and then run off through the sponge... so the same
with the strike energy.
The polyphaser website has tons of info, much of which came from their own
investigation on the subject. I've used it... it works!
73, Stan
----- Original Message -----
From: _HansLG@aol.com_ (mailto:HansLG@aol.com)
To: _towertalk@contesting.com_ (mailto:towertalk@contesting.com)
Sent: Monday, September 01, 2008 2:56 AM
Subject: [TowerTalk] Fwd: tower grounding
Hi Dragonsong,
I believe the rebar cage will be totally isolated from ground as it is
surrounded by concrete. The concrete is a relative good electric insulator
and
will therefor not be good for grounding the tower. You have to add grounding
rods outside the lump of cement holding your tower in place.
There is plenty of information of the "best way" to ground the tower. Two,
three grounding rod driven 8 feet into ground is, I believe, considered
acceptable grounding for lightning strikes. When it comes to RF grounding
the field
is open for suggestions. Little depending on the surrounding ground I,
myself, believe in radials stretch just above or on the surface of ground.
Other
folks believe the radials have to be buried or left several feet above
ground.
The length and amount of radial is a science by itself and I think the
advice is:" The more the better", up to some point that varies between 15
to 60
radial.
Have fun with the tower and stay safe. K1TTT has some serious safety
warnings on his web site that you should look at.
73 de,
Hans, N2JFS
____________________________________
From: _dragonsong@sbcglobal.net_ (mailto:dragonsong@sbcglobal.net)
To: _towertalk@contesting.com_ (mailto:towertalk@contesting.com)
Sent: 8/31/2008 11:06:21 P.M. Eastern Daylight Time
Subj: [TowerTalk] tower grounding
First I want to thank all of you for the info so freely dispersed on this
reflector. I've learned a lot in just the month I've been getting the
discussions.
Now then, I just today made a deal for my very first tower. It seems to be
a
bit over a Rohn 25 size but not quite a Rohn 30. It's about 60 feet in
length and will be bolted to the house and set in a rebar and concrete cage
footing.
My questions are these, is it possible to use the rebar cage as a part of
the grounding system for this tower? And, if so, what is the best manner
of
making that grounding connection?
Thanks up front for your consideration and responses.
Tearlach de w8sco
There's nae man sae deif as he tha' winna hear.
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