[TowerTalk] [Fwd: Protected Land Area]

[Lowell, Mark]

Edward Soriano M.D., DU1OZ, wrote:
>My newly installed 40 foot crank up tower is more or less 280 feet away
>from a 120 feet tall wood post holding a VHF omni commercial antenna.
>Will this high structure protect in a way my crank-up from direct
>lightning hits?

Ed, I will take a stab at a direct answer for this one. One popular
guideline is that a lighting rod provides a 'cone of protection' for
objects below it.

The rod must be connected to a 'good' grounding system to be effective!

Imagine a cone, pointing up, with the tip of the lightning rod at the apex
of the cone. The apex angle is 60 degrees, I think. Sometimes, I think 90
degrees is used. Somebody help me out with this one.
 On with the more conservative 60 degree cone... The sides of the cone
project down at angles of 60 degrees below horizontal, forming an
equilateral triangle. Objects falling inside that triangle will be
protected. See http://www.lightningrod.com/how.html for a neat
illustration of this principle.
 Here at our explosives processing plants, this is accomplished by a
series of tall masts, usually 4, that are placed at each corner of the
building. The cones of protection overlap, protecting the building.
Sometimes a wire is strung between masts.

Ok, on to your example. Plot your masts on graph paper, one foot per
block. I did this already in a CAD drawing program. Unfortunately, your 40
footer is way outside the cone of protection provided by the 120 foot
mast. That means your 40 is still at risk.

Hope this helps,

--...MARK_N1LO...--

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