A local friend ham got hit on a 2m antenna atop a one story house, and that
was less than 38 feet. Certainly at 38 feet one should take precautions,
such as static buildup dissipator atop the antenna mast/tower. Ground the
legs of the tower with grounds that can dissipate large currents, (Crowfoot
grounds to spread the energy to surrounding earth). It also depends on how
well grounded taller objects are. Very green trees in summer probably
would have a cone of protection around them, that is less to not existent
when the sap is down in cold winter, and they are bereft of foliage.
A 50 foot power pole got shattered halfway down across the parking lot from
our 65 foot tower at the club station. There was no harm done to anything
on the tower, but we did get a power line surge that took out MOVs in a
couple of Astron power supplies in the club shack. There was a defective
ground wire on the power pole, the strike went down the pole until it hit a
45 degree angle guy wire and followed that to earth. Everything above that
point was toothpicks. The tower is ground rodded and bonded to other water
pipes underground.
Lightning depends not only on height of the object hit, but on its ability
to bleed off static buildup that leads to the initiating event, if not bled
off. Local earth around two conductive elevated objects can also be a
factor.
-Stuart
K5KVH
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