[TowerTalk] Porcupines and other wives tales

[Keith Dutson]

I should have phased it differently to say there was insufficient potential
to cause a strike rather than draw an arc.  At any rate, it is a THEORY.  I
have not seen any other theory to explain this.

73, Keith NM5G

-----Original Message-----
From: towertalk-bounces at contesting.com
[mailto:towertalk-bounces at contesting.com] On Behalf Of David Robbins K1TTT
Sent: Tuesday, July 04, 2006 4:45 PM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Porcupines and other wives tales

Cosmic rays super charge the cloud?!?!  Now that's a new one.  the last time
I read a journal article about cosmic rays they were cited as a possible
cause of the initial breakdown because they left a trail of ions.  there is
no way they can contribute to the overall charging of the cloud... consider
this, a cosmic ray is a single highly energetic particle that is entering
the atmosphere.  It leaves behind a trail of ionized air, much like a meteor
trail but much thinner.  This trail has no bulk charge, it is only a very
local effect where some electrons got bumped off the atoms that the cosmic
ray interacts with.  there is no net charge introduced or moved any great
distance, just a very short lived, very narrow trail of ionized air.  This
could be enough to help steer or maybe help initiate the downward leader if
it happens to penetrate a charged area in the cloud.

You also must remember, that the millions of volts often quoted as the
potential of the stroke is not a measure of the breakdown voltage from the
cloud to ground.  lightning is NOT like 'drawing an arc' with a welder or
other high voltage source over a short distance.  The stepped leader
progresses because the electric field near it's end gets high enough to
ionize the air right at its tip, once that breakdown starts it jumps maybe
50-100m, the distance of each hop is related to the total charge in the tip
of the channel.  once the electric field at the tip drops enough so that the
air doesn't ionize any more it stops, more charge gets pushed down the
channel and the field builds until the process repeats.  


David Robbins K1TTT
e-mail: mailto:k1ttt at arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
 

> -----Original Message-----
> From: towertalk-bounces at contesting.com [mailto:towertalk- 
> bounces at contesting.com] On Behalf Of Keith Dutson
> Sent: Tuesday, July 04, 2006 20:04
> To: 'Jim Jarvis'; towertalk at contesting.com
> Subject: Re: [TowerTalk] Porcupines and other wives tales
> 
> >The measured energy of the strike is not the point here. The point is 
> >to
> try and prevent the charge build up to where the strike occurs.
> 
> Well, if you are going to prevent a strike, you basically have to 
> bleed off all of the strike energy.  I don't think that is possible.
> 
> A cloud that is charged by the conventional rain drop method does not 
> have the potential to form a strike.  This charge has been measured 
> just prior to a strike and found to be far short of the potential 
> required to draw the arc.  The latest theory is that random cosmic 
> rays strike the cloud causing a portion to be supercharged.  The 
> question is now how fast does this happen?  I certainly do not know 
> the answer, but if it is a matter of seconds, there is nothing that 
> can bleed off the charge and stop the strike.
> 
> 73, Keith NM5G
> 
> -----Original Message-----
> From: towertalk-bounces at contesting.com 
> [mailto:towertalk-bounces at contesting.com] On Behalf Of Jim Jarvis
> Sent: Tuesday, July 04, 2006 1:59 PM
> To: towertalk at contesting.com
> Subject: [TowerTalk] Porcupines and other wives tales
> 
> 
> 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.
> [snip]
> 
> _______________________________________________
> 
> 
> 
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