" Personally for #3 - I take the strategy that on a ham's budget, we are never
going to do a good job at a major strike. I am just going for not exploding
the tower base. So I put 3 ground rods in the hole itself with direct
connects at the bottom of the buried base section. So that at least there
is no concrete path of choice to ground on a direct hit. Beyond that I
assume the ground itself will dissipate the energy just like any other
forest strike."
as far as this one... it is extremely rare that lightning explodes a tower
base. Just think of how
many times high voltage power lines get hit every year and how few reports of
tower foundations being
damaged there are. And then go find all the pictures hams aren't posting about
their poured by hand,
ungrounded, no rebar foundations having been blown up by lightning... no, not
everyone listens
to the prime directive and does what the manufacturer says... heck, i'll admit
it, I have a tower here that
has a foundation like that... well, except it does have one rod next to it that
is attached to the house
perimeter ground.
now, about rods at the bottoms of the legs... first of all, 3 rods in the same
foundation hole aren't going
to be far enough apart to be considered separately so they aren't much better
than a single rod. then,
keep in mind two things. the diameter of the tower legs is bigger than the
rods and they are probably
connected to the rebar either by design or accident, and the conductivity of
the concrete is higher
than the conductivity of the soil. so, the current comes down the tower into
the foundation, sees the
nice big fat ufer ground and since most of the opposite charge is near the
surface because it was
attracted by the leader coming down from above it heads out through the
concrete before it even
gets to the rods at the bottom that have a higher impedance. there have been
some published
experimental results, and algorithms, (and unfortunately more that are still
proprietary or copyrighted
and still paywalled) showing that the current from a vertical rod or foundation
mostly goes out radially
and not off the tip. also,the current does not flow uniformly, it usually
forms streamers though the soil
(google 'fulgurite' for extreme examples) so you can expect that there will be
lots of streamers through
your concrete, or just across the top of it, as the charges rush towards each
other. my preference is
to go from the tower legs to a rod near the foundation just as a way to spread
out the current a little
bit more than the ufer ground will by itself, though the effect is small
electrically, its effect on inspectors
and visitors is large.... especially when they see the huge conductor I used,
though most don't seem to
notice that the conductor is fatter than the cross brace it is clamped to or
the rod itself.
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