[geeks] Whee! Lightning strikes, AGAIN!

[gsm at mendelson.com]

On Wed, Jul 29, 2009 at 02:28:41AM -0400, Shannon Hendrix wrote:

> You are talking about a lightning dissipator.  They are ungrounded,  
> pointed rods in most cases, but some designs are spiky or even blunted.  
> The theory is that they will bleed off charge and reduce the potential 
> difference between ground and cloud.


If you are referring to me, I said something completely different. I said
that they were grounded. I pointed out that the one I used was a ham radio
antenna of which the entire thing was DC grounded.

The ground part was important as at 144mHz, the antenna was not grounded,
it had an impedance (RF resistance) of 50 ohms. Not relevant to the discussion
so only DC ground was mentioned.

I was also very specific (but possibly obscure) in my description of one,
a ball with lots of little things sticking out. Ok the illustration I
used has feet, but most people would get my point. I apologize to the ones
who either did not, or did not not know the cartoon character.

> The lightning research community generally rejects the idea and further 
> says that it increases strike probability rather than reduce it.

I'd love to see that. Everything you say about ungrounded dissipater is
correct, but where do you see that for grounded ones? Maybe you did not
and lost in all the noise was the point I was referring to grounded ones.


> Most of what is sold out there is pseudo-scientific crap.

I'll agree with that, my first comment on the subject was about surge
protectors and how MOV's are exactly that.

> A lightning rod is, by contrast, designed to intercept a strike and  
> direct it to a controlled ground.  The theory here is to channel it  
> through a safe path to prevent it from channeling where it will do harm.  
> All lightning rods are grounded to earth or water, sometimes via a 
> grounding network or a building frame designed for that purpose. Some 
> earth grounds used are salted underground cable networks.


Since Franklin's time, it has been shown that except for rare occasions
(mostly existing to make the WikiPedia seem fair), they cause lightening
strikes. While it's a great idea to channel them safely, in Franklin's
day the EMP problems caused by them were of no consequence. 

In modern times, with EMP being a serious problem (and the subject of
the original question), a lighten rod must be understood for exactly
what it does, and how it does it. 

Therefore putting a lightening rod on your home may safely channel the 
lightening and avoid having the house burn down Franklin BTW started
the first fire brigade and fire insurance company and penned a play
about it called "A Truth With No Convenience" (just kidding about the play).



> The lightning rod creates a field above the rod which intercepts nearby 
> strikes by acting as an upward streamer channel which connects with 
> cloud->ground stepped leader and connects that to a controlled ground.  
> This helps prevent the strike from going through structure, people, 
> electronics, etc.

There you go mixing intercepts (downward) and creating upward streamers.
Guide or attract would be a much better, IMHO word. 

Geoff.

-- 
Geoffrey S. Mendelson, Jerusalem, Israel gsm at mendelson.com  N3OWJ/4X1GM