From: gary@ke4zv.uucp (Gary Coffman)
Subject: Re:  metal tower vs. non-conductive tower
Reply-To: gary@ke4zv.UUCP (Gary Coffman)
Organization: Gannett Technologies Group
Date: Fri, 13 Nov 1992 18:21:31 GMT

In article <9211121003.aa02197@cbda7.apgea.army.mil> wejones@cbda7.apgea.army.mil (Bill Jones) writes:
>> 
>> I have been looking for a 40' tower for a while and came across a
>> company which sells a really sharp 40' fiberglass pole. Originally
>> it was designed for use as a street light but looking at the spec
>> sheet it appears it should perform well as an antenna tower. I'm
>> interested in using it since it's not that expensive compared to an
>> equivalent steel tower, and it is MUCH, MUCH more attractive.
>> 
>> The question is: Is there any positive or negative impact in using
>> a non-conductive tower over a steel one of the same dimentions?
>> 
>> BTW: I want to perch a beam and rotor on top of the tower.
>> 
>>
>There has been some friendly debate around here concerning whether
>a well grounded metal tower makes it more or less likely to
>suffer lightning strikes.  There is no argument that a well
>grounded tower would reduce the damage once lightning did strike.
>The prevailing opinion of some of the experienced oldtimers is that
>a well grounded tower "makes the tower invisible to lightning".  I,
>myself think that a well grounded tower would be a much shorter path
>to ground, and thus much more likely to attract lightening.

A well grounded tower, with an adequate dissipation array on top, 
can *reduce* the chance of a strike, but in most cases, a tower of
any type becomes the highest object in the area and is *more*
likely to be struck than other objects. Since the charge buildup
that causes lightning strikes is a *static* field, a fiberglass
tower is no protection from lightning strikes. It may be an even
greater danger than a metal tower in it's ability to hold a charge
that will attract lightning. After a strike, the fiberglass may
act as an impediment to the *flow* of current to ground, but at
the voltages present in lightning strikes, the most likely outcome
is that the fiberglass will be *shattered* as it's insulation is
*broken over* by the very high voltage between cloud and ground. 
This commonly happens with trees that are struck by lightning.

A well grounded tower offers the best protection for an area from
lightning strikes. It will be the best path to ground, and will
divert most of the current away from other structures and equipment.
If the tower is not properly grounded, it can cause more damage
to other equipment connected to it, or near it, than if there were 
no tower present. But if it's grounded *right*, and that's not simple, 
then most equipment will not be harmed when the tower is struck. 

I took a lightning strike on a 70 foot metal tower that was set 8 feet 
in the ground in concrete. I suffered over $10,000 in damage to equipment
ranging from telephones and lights to radio and computer equipment. Since 
then the cobbler has fitted his children with shoes and I've taken multiple 
strikes with *no* damage to equipment. If the procedures in the NEC and the
NAB Engineering Handbook are followed, lightning strikes can be controlled.
The key is to avoid current loops in the grounding system.

Gary KE4ZV
Newsgroups: rec.radio.amateur.misc