[TowerTalk] Porcupines and other wives tales

[Jim Jarvis]

I had tuned out on the water tower-omni antenna lightning protection,
and noticed the "porcupine" post.  Things morphed somewhat, I see.

To avoid forensic reading and proper attribution, let me say that
some folks have said some right things....and a few may underestimate
the problem or over-estimate the potential for remediation.

consider this exchange:

Just look up measured energy
> of
> a strike and compare it to the ability of the porcupine to dissipate this

The measured energy of the stike is not the point here. The point is to try
and prevent the charge build up to where the strike ocurrs.

-0-
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.

Be glad you're not on a sailboat at sea.  That problem is worse.  I've been
in the middle of a
super-cell, with cloud to cloud lightning arcing above us over a 10 mile
distance...instruments
and nav lights off...and all were blinking and banging full scale with the
strikes.  Subsequently
decided to keep a change of knickers in my sailing bag.

N2EA