Grounding tower legs

[DAN]

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
> 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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