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[TowerTalk] Lightning 'porcupines' & rods

To: <>
Subject: [TowerTalk] Lightning 'porcupines' & rods
From: (Mark L.)
Date: Thu, 19 Aug 1999 12:36:01 GMT
Brian wrote:
<< 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>>

Hello Brian,
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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