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[TowerTalk] Lightening Strikes and Ground Conductivity

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Subject: [TowerTalk] Lightening Strikes and Ground Conductivity
From: (Jim Reid)
Date: Sun, 1 Aug 1999 13:15:05 -1000

Interesting discussion occurred on yesterday's
Hawaii Afternoon Net.  For the most part,  very
few strikes of lightening occur on the HI islands.

About Kauai,  when large thunder storms come,
which is about once every 18 months or so,  the
strikes always seem to be just off the coast and
into the ocean water.

U of H reports that Hawaii soil -- decomposed
volcano "effluent" -- has very poor conductivity,  and
high dielectric from the decomposed glassy
lava flows.  Now sea water has a conductivity
of about 50 deci-siemens per meter.  U of H
reports that typical Hawaii soils should measure about 
1 deci-siemen per meter in undisturbed rain forests, 
about 2 deci-siemen per meter in agricultural areas, 
and 3 deci-siemen in areas like residential lawns or 
even higher, based on the increasing augmentation 
by extra organic material or even chemical fertilizers 
vs the undisturbed rain forest floor. 

Further,  what soil there is,  is very shallow before volcanic
"blue stone"  or lava flow rock is struck;  one example:

Dean Manley,  KH6B reports:

"I installed KAHU [1060 Khz. AM] in Hilo [Big Island] over 
15 years ago. Its ground system was installed by hand, 
pick and shovel. Five miles of #10 solid copper wire 
installed as 120 radials, covering 5 acres",  [of essentially,
lava rock!].

"The result was one of the more efficient BC stations in 
Hawaii, especially on this island. The FCC wanted the 
station to lower the power from the 1 kW specified in the 
construction permit to 938 watts. The measured 190 mV/meter 
at one mile exceeded the expected 175 mV/m.

When I reminded the FCC about the poor ground 
conductivity in Hawaii, they said "OK" to the 1 kW. 
My measurements showed that the average ground 
conductivity 0-20 miles from Hilo to be less 
than 1 mmhos/m (not including sea water paths)."

So far Hawaiian amateurs'  experience is that our ground
is very poor,  verticals don't work much at all using less
than ideal grounded radial systems,  and lightening
strikes on installations are essentially non-existent.

The lightening bolts,  what few are seen,  go to the sea
water,  not the trees or towers,  etc.  on the islands.
Can this be because of the high resistance of our
soil,  and very shallow depth of what there is of it?

We decided that was the answer,  but is it so??
Maybe we are just lucky, plus the infrequency of
such storms out here.

73,  Jim,  KH7M

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