Well stated.
I have had a 'pointed' mast without a porcupine and a
'pointed' mast with a porcupine and in both configurations I
have not had any lightning damage ever, anywhere at my
present QTH in almost 30 years. My neighbours say that they
have seen my tower take a direct hit, out of the corner of
my eye I think that I have seen my tower take a direct hit,
but I can not absolutely prove that indeed the tower took a
direct hit. The top of my main tower will bleed off the
ground charge...it is very audible. I am not promoting or
discounting the porcupine, just sharing my experience, and
my experience doesn't cost you anything.
Doug/VA5DX
Subject: [TowerTalk] Porcupines and other wives tales
The point (no pun intended) is to bleed off the
charge faster than the ground charge builds, whether it be
moving into the
area, building up, or both. If that charge is building on
the object faster
than it can be bled off and a feeder is produced that
connects then the
object will be hit. When that strike ocurrs the current
comes from an area
considerably larger than the tower and contains far more
energy than would
have been available from the tower.
-0-
It's all well and good to try and bleed off charge via
lightning rods...even
'fattened' ones like porcupines. But those rods have a
fixed ability to
handle
the discharge of an approaching charged cloud. Lifetime of
a porcupine with
a
direct strike is measured in nS.
Fast moving, highly charged clouds WILL overwhelm them.
There WILL be a
strike-leader,
and then a main strike. It's only a question of when and
how often.
There is a reader of this reflector, who has observed
approaching squalls
with strikes
cease striking, as they approached his multi-tower antenna
field. Even he
wouldn't assert
that he'd never been struck because of the extensive
protection system.
There are too many variables in this problem to assert
anything as
certain--except, in my
opinion, the liklihood that you'll be struck if you wait
long enough. Will
a topside lightning
rod..or multi-rod-porcupine statistically reduce the number
of
strikes...probably. Will it be
material? Ya pays yer money and takes yer chances.
In my view, the upper antenna and all coax and control
cables on the tower
are sacrificial.
Get the tower as far from structures as you can...ground
well at the tower.
You can protect from
'secondary strikes', or induced current from a nearby
primary strike, but
that's about it.
After that, it's just cleanup.
N2EA
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