k4sb@avana.net wrote:
>
> ..................
>
> We should also keep in mind that lightning originates from the ground
> UP, not the way we see it. I would assume therefore that as long as the
> tower is held at ground ( whatever that magic number is ), it would be
> impossible for it to be hit...............................
>
> Name: ed sleight
> E-mail: k4sb@avana.net
To: <towertalk@contesting.com>
> Date: 11/7/96
> Time: 11:09:31 AM
>
Ed,
Conventional lightning theory holds that an excellent ground will not prevent
lightning
strikes. The purpose of having an excellent ground is to control the path of
the
current (as much as is possible), so that most of the current is directed away
from
cables and equipment that are attached to the tower. With a good ground, the
potential
across your feed lines might be held low enough to minimize damage inside your
shack.
A good source of basic technical information is the Polyphaser "The Grounds for
Lightning Protection" booklet. They explain that the tower's inductance is the
primary
reason for the high "top to bottom" voltage potentials during a strike (not the
joint
resistance). You can even estimate the voltage potentials that your system is
likely
to see in the event of a strike.
Prevailing theory suggests that the height of an object is the primary factor
in "hit
frequency". Whether the object is a tree (poor conductor) or a tower (good
conductor)
is not a significant factor. Hit frequency is so low for "low objects" such as
a 50
foot tower for example, that one person's individual experience is not a good
indicator
(sample variance is too high). Also, a tower with an excellent ground may
experience
hits that the owner is not even aware of.
I've had several hits on my 20' TV mast--it has an excellent ground, and other
than a
blown feedline, none of the damage has yet entered our home. (This is no
guarantee for
the future, because some strikes are as much as 10 times more powerful than
others.)
By the way, here in Florida, it is not uncommon for lightning to penetrate
several feet
of dirt to hit an underground power cable conduit. This is obviously "low" and
well
grounded, but it still gets hit!!
I thought you might be interested in a few of the tidbits I've acquired after
several
years of lightning study.
73,
Dan - AD4HW
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