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[TowerTalk] Length of Mast

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Subject: [TowerTalk] Length of Mast
From: (David Robbins K1TTT)
Date: Sun Jun 1 16:56:32 2003
> Years ago I heard W0UN's presentation at Dayton where he claimed that
> a thunderstorm would move over his multi-tower setup in Colorado that
> lightning would actually stop until the storm front would move beyond
> antenna farm. The towers were heavily grounded as I recall, but I
> remember what else he did for lightning protection. I don't think he
> much undone on that front. This would support the theory of lightning
> to prevent lightning strikes. And that is what I learned years ago in
> college meteorology class.
nothing you can do on the ground is going to do anything more than
slightly attract a downward moving leader from a cloud once it gets
started.  You can do this by providing a large conductive area like a
power line that increases the charge concentration on the ground.  After
that the only control you have is to help initiate the upward streamer
from an object that you would rather get struck than something else in
its immediate vicinity.  That is where lightning rods come in, they
promote the early initiation of that upward streamer that connects with
the downward moving leader to trigger the stroke. 

To give you a feel for how small of an effect man can have on lightning,
lightning is attracted slightly toward high voltage power lines.  For a
typical 20m tall structure with 3 power conductors and 2 shield wires
this attraction will pull a 25ka stroke in towards it about 10m from
where it would otherwise hit the ground.  After traveling probably
thousands of meters from the cloud, a relatively large conductive
structure has only a tiny effect.  Compare this to the striking distance
(the length of the upward streamer when it connects to the downward
leader) of about 70m and you can see that even a large metallic
structure has only a very small influence on the location of the stroke.

David Robbins K1TTT
AR-Cluster node: 145.69MHz or telnet://

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