Many years ago I purchased a porcupine based on a testimonial. Now that I
have facts to dispute that testimonial I no longer need it. If anyone out
there wants it, it is yours free. If you live near the Houston area you can
pick it up. If not, I will send it to you if you agree to pay the postage.
73, Keith NM5G
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Doug Renwick
Sent: Tuesday, July 04, 2006 2:56 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Porcupines and other wives tales
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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