David,
I would venture to say there are more insulated, serial fed towers in
the air, than grounded
through the foundation amateur towers, by a fair margin. But I
could be wrong, and it's totally
academic.
You are right that a good conductor through the concrete represents a
shunt path. So long as it
has the current carrying capacity, and isn't degraded by corrosion,
it's better than not.
I think, reading what you wrote, that you would agree that having
that shunt path OUTSIDE the
concrete is preferable. We don't differ on the question of brute
force grounding. But I believe
strongly .. STRONGLY ... that the routing of the current path
through the concrete is equally
important. i.e. no right angles, gentle bends, lots of surface
area from copper strap, etc.
I've seen what corrosion happens with copper pipe in concrete, after
15 years. All the heating pipes
in my VT house were run inside a slab, and not inside pvc conduit.
For the longest time, I wondered
why my basement floor was pleasantly warm in the winter, until I
realized we were leaking hot water.
And where the copper tube for the oil input came through the
foundation? It, too, became leaky.
So, while I will accept arguments supporting in-concrete ground
systems, when they're first installed,
I will remain skeptical of their downstream value. I want my
lightning protection grounds to be where
i can see them and maintain them, if required.
n2ea
On May 29, 2008, at 2:03 PM, David Gilbert wrote:
>
> Per your comments, the two towers you refer to were isolated from
> the concrete base. That is the same condition described by
> Polyphaser for the single semi-verified claim of an exploding base
> they could find. If the lightning path, which can be capricious if
> not channeled (and even if it is channeled), decides to make its
> own unique path to ground through the concrete, there is a good
> chance it can cause it to explode. I was able to find at least a
> few pictures of concrete structures (bridges, roads, etc) that had
> been damaged by lightning in that manner. Most towers don't fit
> that category, though. The tower base or supporting bolts for most
> ham towers are embedded well within the concrete, and as you say in
> that case (which means in most cases in my mind), brute force
> bonding everything together and providing any and all practical low
> impedance shunt paths paths to ground around or through the
> concrete would seem prudent. What some people don't seem to get is
> that a wire through the concrete is in fact a shunt path to the
> bulk of the concrete.
>
> 73,
> Dave AB7E
>
>
> jim Jarvis wrote:
>> David,
>>
>> Urban legends? I don't think so.
>>
>> WCTC-AM, New Brunswick, NJ. Took multiple strikes, and had
>> structural cracks
>> in the foundation afterward. This was in the 1965's, when I
>> worked there as an engineer.
>>
>> WBRW-AM Somerville, NJ. 4 tower 190' inline array. One tower
>> took a strike, during
>> construction and proof of the array. A crack appeared in the
>> foundation, from the insulator
>> base to earth, opened to a width of 1", showing burn marks. 1972.
>>
>> Both of these were series fed sticks. i.e. insulated, with
>> single- point tower bases.
>>
>> In the interest of length, I removed a paragraph from my post,
>> which said something like:
>> "The area of debate probably occurs when the tower mounting
>> itself routes lightning into
>> the foundation. i.e. J-bolts or tower legs extend into the
>> concrete. In that case, an argument
>> can be made for the brute force method of bonding everything
>> together."
>>
>> Guess I shudda left it in, eh?
>>
>> You're right about cadwelds.
>>
>> As for my argument that you want to route lightning around the
>> foundation as much as possible,
>> I'm sticking to it. In addition to two broadcast arrays, I've
>> seen enough exploded sidewalks from lightning,
>> I'd rather not dispel the urban legend myth with first-hand
>> experience.
>>
>> It is my contention that the shape and form of the re-bar does
>> NOT constitute a low-impedance path to
>> the complex lightning waveform. It is highly inductive. Tie
>> it together? fine. But I'd bypass it with
>> 4" copper strap, (or 1.5", if that's what you're using), making
>> a nice, smooth 18" radius bend, and exiting
>> the concrete away from the rebar.
>>
>> Wish I had digital pix of our ground treatment of the 500' WCTC-
>> FM self supporting tower. I was consultant
>> on that project in 1978. It gets hit in virtually EVERY storm.
>>
>> N2EA
>>
>>
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