<< I've seen several posts in the past praising the virtues
of 'porcupines' to 'drain off charge' before a strike can
occur. I've also seen at least as many claim it is all
bunk. I must admit it seems reasonable, but not if a
lightning rod 'attracts' strikes>>
Lightning abatement seems to be at least a two step approach:
1) Provide dissipation ("porcupines")
2) Provide a preferred strike current path ("lightning rod"/"air terminal").
Dissipation helps prevent strikes by dumping excess ground charges into the
air via ionization. Lots of small, pointed conductors do this well, but the
small conductors cannot take strike current in the case that the ground
charges are large enough and a strike does occur.
Both steps depend heavily on a good ground system for your tower. There is
plenty of info in the Towertalk archives for this. Don't skimp on the ground
For step 1, I adopted an old timer's trick, described in a very old
Towertalk post: using guy wire tails as dissipators. I left 3 feet of excess
guy cable (EHS) at each guy attachment point to the tower. I untwisted it,
and clamped it to the adjacent tower leg, such that each strand has intimate
contact with the tower leg and the hose clamp. I then twisted the wire back
together, except for the last 8 inches, which I fanned out into a "porky". I
cut each end of diagonally, leaving a sharp point, and pointed the
improvised dissipators up and out at 45 degrees toward the sky. Since I have
3 sets of guys, this gave me nine dissipators for $0 and my time in
preparing them (takes a while to do).
For step 2, I adopted a method that the local cable TV company uses to
protect their towers. I installed an 8 foot, copper-plated ground rod, point
up, on the tallest point of my tower, which was the mast (the cable
company's towers don't have masts).
Since I have an aluminum mast, I used a piece of stainless steel foil
between the aluminum and the copper to prevent corrosion, and clamped it
with several stainless hose clamps.
Now, should the disspators get overwhelmed, the ground rod should take the
strike harmlessly, instead of the poor fiberglass vertical, which usually
gets shattered rather remarkably in this typical roost.
Hope this helped someone. Good luck with your project!
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