Jerry Keller, K3BZ, wrote:
> My 40' tower will have a 12 foot mast. I've heard it's a
> good idea to fasten a sharp-pointed rod at the top of the
> mast... supposedly to reduce the risk of a lightning
> strike....like the lightning rods I see on barns. If it's
> true, does it really help a lot? (I don't notice a lot of
> other hams doing it, so I'm wondering)
Back in the early 1980's, there was an interesting article
published in the "Journal of the Franklin Institute" on this
topic. While this journal is normally devoted to advanced
automatic control theory and theoretical electronics, they
have historically published many articles on lightning
research.
The article was written by researchers at Langmuir Labs of the
University of Arizona where there is a significant research
program on lightning and its abatement. In the article they
showed an analysis where they proved that the optimum shape of
the end of a lightning terminal (what we normally call a
lightning rod) is rounded in the form of an oblate spheroid
(like one end of a football).
This conflicted with the National Electric Code at the time
and it caused quite a bit of controversy. But the Arizona
researchers countered with experimental data on hits to over
5000 different pointed lightning terminals. By far the
largest number of hits occurred not at the point, but at
locations a few inches to several feet below the pointed tip.
So the sharp pointed rod is not necessary, and is, in fact,
less than optimum.
For more details, consult the following web sites:
"LIGHTNING ROD BEHAVIOR : RECENT INVESTIGATIONS",
National Lightning Safety Institute,
http://www.lightningsafety.com/nlsi_lhm/lightning_rod_recent.html
"Lightning misses the point",
Nature Magazine,
http://www.nature.com/nsu/000518/000518-5.html
"Protecting Your Station From Lightning",
National Association of Broadcasters,
http://www.nab.org/membership/benefits/Nov00.asp
"The Basis of Conventional Lightning Protection Technology",
Report of the Federal Interagency Lightning Protection
User Group,
http://www.lightningsafetyalliance.com/images/The%20Basis%20of%20Conve
ntional%20Lightning%20Protection%20Technology.pdf
73, Dr. Barry L. Ornitz WA4VZQ ornitz@tricon.net
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