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Re: [TowerTalk] CATV & Phone grounds

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Subject: Re: [TowerTalk] CATV & Phone grounds
From: "K8RI on Tower talk" <>
Date: Tue, 11 Jul 2006 16:44:56 -0400
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> On Tue, 11 Jul 2006 14:29:33 -0500, you wrote:
>>Rather than thinking of lightning as an electrical current flowing from
>>ground to cloud or vice versa, I find it more interesting to think about
>>what it is composed of: plasma.  The free electrons and ions created by 
>>plasma must be neutralized by some method.  This is where current flow is
>>involved.  Pick a path and connect via some conductor (tower, copper wire,
>>tel wire, power line, water pipe, etc) to facilitate the neutralization. 
>>is fairly easy to assume current will travel only one way on the 
> ------------ REPLY SEPARATOR ------------
> As I understand it, plasma (in the general sense) can exist with or
> without current flow.

Plasma can be created with high temperature, voltage, or both as you do have 
a high temperature once the electrons are stripped off the atoms and 
molecular bonds broken.

> In the absence of external heat, plasma will

The only difference I'd use the word "energy" instead of heat.

> quickly recombine into normal atoms, but while it is in the plasma
> state, it will conduct much like a wire would.

This is one of those "Kinda, sorta" things and it does help to think of the 
plasma like a conductor, or conduit to support the initial step leader and 
provide a low enough resistance for the voltage to "flash over" to create 
the strike.  The "step leader" forms a plasma conduit albeit neither the 
straightest of most continuous. Note how the lightning flashes off to the 
sides of the main strike or how some consisting of many lines spread out 
over large areas.

Nor is the step leader a low resistance like we'd expect to find in a wire 
or most conductors.  At these voltages and environment every thing becomes 
relative to a point.  Initially the step leader is a relatively high 
resistance, weak plasma (Ionized air).  However the resistance is low enough 
that the initial strike can "arc over".  This produces a much hotter plasma 
and much higher density (very high pressure for a short duration) and much 
lower resistance which provides a path that requires much less voltage for 
an arc to be established.
That in turn allows more, but weaker charge from the clouds to be drained in 
repeated strikes.

I don't have the figures at hand for the pressure inside the initial bolt 
but it is quite high.  Temperatures are phenomenal.

If you've ever used a "plasma torch" for cutting metal you have at  least 
the beginnings of an  idea as to the temperature inside a lightning bolt. 
The typical "torch" uses low pressure air and a relatively low power arc to 
generate a plasma of roughly 30,000 degrees.  This is hot enough to vaporize 
thin sheet steel so quickly I have cut painted "barn metal" without 
scorching the paint either side of the cut.  Even with a smaller torch and 
using a guide I've cut 1/4" steel plate more smoothly than I could have done 
in a band saw and I'm a rank amateur when it comes to the torch.  However 
IIRC lightning can produce temperatures many times this. I believe some 
strikes are capable of producing something on the order of one to two orders 
of magnitued more which is hotter than the surface of the Sun.

> In the case of lightning, the enormous voltage present rips the air
> molecules and atoms apart, creating the plasma, which then provides a
> current path for the lightning strike.
> If my understanding is wrong, please enlighten.

Close enough for government work <:-))  The short version (ignoring the so 
called super strokes)  is the step leader forms a weak plasma allowing the 
initial stroke to arc over, which produces a much stronger plasma to support 
the following strokes. Most of the HF component is due to the rise and fall 
times of the strokes.

Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2

> Bill, W6WRT
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