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[TowerTalk] Length of Mast

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Subject: [TowerTalk] Length of Mast
From: (W0UN--John Brosnahan)
Date: Sun Jun 1 18:15:45 2003
At 03:28 PM 6/1/2003 -0500, Jim Rhodes wrote:
>Years ago I heard W0UN's presentation at Dayton where he claimed that when 
>a thunderstorm would move over his multi-tower setup in Colorado that the 
>lightning would actually stop until the storm front would move beyond the 
>antenna farm. The towers were heavily grounded as I recall, but I don't 
>remember what else he did for lightning protection. I don't think he left 
>much undone on that front. This would support the theory of lightning rods 
>to prevent lightning strikes. And that is what I learned years ago in a 
>college meteorology class.

Although I was involved in lightning research in the 1970s on
tracking the movement of the stepped leader by interferometric
positioning of its low-VHF radio emission (published in the Journal
of Geophysical Research--forget the exact issue) this note is not a
"scientific" comment.  Only a reiteration of observations in Colorado.

It was my experience, confirmed by K0RF and W4ZV (then W0ZV)
that a grounded, naked tower would act as a point discharge and
tend to attract lightning.  But a tower with a large number of grounded
Yagi elements, such as stacked arrays, would tend to drain the
charge from the area around the tower and greatly reduce the occurrence
of lightning in the immediate vicinity.

It is my experience that a single lightning rod at the top of the tower
will have no added real effect--if the tower is "naked" it is more likely to be
struck, with or without the lightning rod than if it is covered with
grounded elements.  And in fact, a large number of grounded elements
seemed to greatly reduce or even eliminate lightning to the structure.

And those little "spiky" things provide no significant improvement over
static discharge than a single-pointed rod.  The volume that the spikes
cover is just too small to make much difference.

K0RF, W4ZV, and myself all noted that electrical storms would stop
striking as they approached the grounded towers (with lots of elements)
and then resume striking as the cell passed by the towers.  Typical
experience indicated that the lightning would stop some 1/8 to 1/4
mile away and resume in an equivalent distance once the cell had passed.

My CO qth was hit a number of times, power lines, phone lines, vertical
antennas, naked towers, etc--but once I installed well-grounded towers
with stack and grounded arrays there was never another direct strike.

Five strikes during the three years I had up some short (40 ft) verticals with
significant in-shack and in-home damage.   Once I installed the tall
towers -- 170-200 ft -- with grounded yagis I had no more strikes during the
subsequent seven years.  Other than one strike to a yard light on the
barn that was outside the "cone of protection" provided by the towers.

The barn was about 400 ft from the house and in a low area--in fact the
yard light on the end was actually LOWER than ground level for the
house and towers.  This strike took out some AC wiring in conduit inside
the metal barn.

 From MY personal experience and comments made by others with similar
experiences I firmly believe that a naked tower, with or without a lightning
rod tends to attract lightning.   But that a well grounded tower with lots
of grounded Yagi elements tends to reduce or virtually eliminate strikes
in the immediate vicinity.   YMMV!

73--John   W0UN

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