[TowerTalk] Lightning protection - length of grounding rods

[David Robbins]

" 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.