To: <towertalk@contesting.com>
>Date: Thu, 04 Jun 1998 15:46:36 -0400
>From: "Guy L. Olinger" <k2av@qsl.net>
Snip... for brevity
>
>It bothers me a bit to see some on this server discount
>anecdotal material. If the theory cannot absorb the anecdotal
>material (assuming no bald-faced lies), then the theory is
>short. On the other hand, extrapolating a few anecdotes into a
>pet theory is equally short. But MASSIVE anecdotal evidence
>cannot be ignored for it's practical uses:
>
Hi Guy,
I don't discount anecdotal accounts. I very much enjoy seeing
them. Sometimes, I even inflict them. I am nearly always
educated to some degree by the accounts posted by others. Even
when my evaluation of the account reaches a very different
conclusion about causality or whatever from the one being prmoted
by the poster. I assume others have the same experience with my
anecdotal accounts.
>Lightning rods work (eg, protect the barn) if connected to good
>grounds.
>
True. I have personally observed this.
>Massive ground fields under towers work.
>
Also true.
>Lightning LOVES trees.
>
Lightning loves anything that is conductive (to some degree more
than air) and convenient. By their nature, trees are frequently
convenient. We get the idea that lightning prefers trees because
the effect of lightning on a tree is so easily observed. You
don't have to actually be there when it happened to see that the
tree got hit. If your tower is well bonded and grounded, you'll
never know when it is hit unless you actually see it happen.
>Ungrounded or poorly grounded conductors from the vicinity of
>poorly grounded lightning attractors create fireballs in the
>hall and burn down people's houses (many years of fire
>dept. experience to back this up).
>
Amen! Personal observation on this one too.
> Here, simply disconnecting lines may guard against induced
>surge, but may provide a path toward the house for a nearby
>strike on a tower with a poorly dissipating ground system. Even
>grounding both ends would not help if the connected grounds were
>poor.
>
Very true.
I have no problem with the concept of disconnecting feedlines
from equipment as a substitute for installing commercial gas tube
arresters. However, I think that doing this absent first
bringing the feedline to and bonding it to an adequately grounded
entry panel is dangerous in the extreme. If one must use the
disconnect method, please be sure that there is a good ground on
the feedline between the disconnection point and the antenna.
You don't want to become the path.
>Moving toward massive evidence:
>
>Lightning doesn't strike "porcupines". Any reasonable theory
>must explain why.
>
I have seen lightning strike the porcupine devices on more than
one occasion. One time we had to scramble to get out of the rain
of needles that resulted when the thing that held all the needles
vaporized and released them all at once. Those little pointy
things are dangerous when falling from the 500 foot level on a
tower. We were lucky in that one of our group actually observed
the hit and saw the things falling in time for us to step under
the site roof overhang. Every time we return to that site, we
spend part of the day picking needles up so they don't puncture
tires.
So I don't think any theory is going to be able to explain why
they don't get hit. They do get hit. It would be relatively
easy to formulate a theory to explain why people don't think they
get hit.
>
>I'm putting up a tower shortly. I intend to put in a large
>ground system as soon as the base is poured. What I don't know
>is how to measure the lightning receptivity of the ground. Any
>help on this?
>
I might be able to help with this one.
For the tower base (assuming it isn't located right near the
service entry point for the house):
In most areas it is possible to rent or borrow an earth
resistance tester. With this device you can measure the
connection resistance of a single ground rod. If it is 25 ohms
or less, you can probably do an adequate lightning protection job
with a few in parallel. But be sure to space them at least two
times their length from each other. Around here, a single 10
foot ground rod averages between 25 and 30 ohms.
For the feedline entry into the envelope of the house:
If the house has no UFER ground for the service entrance, but
does have metallic cold water distribution under the slab, we can
normally get the lightning ground resistance down to 5 ohms or
less with a pair of ground rods and the subgrade plumbing tied
together. If it has a UFER in the slab, the service entry ground
is usually down in the 2 ohm range already.
I strongly recommend that you establish single point feedline
entry panel very close to your service entry point and use the
service entry ground. The earth resistance meter can be used to
evaluate the quality of your service entry ground. If it is 5
ohms or less, use it. IF not, enhance it with ground rods and
use it.
73, Eric N7CL
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