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[TowerTalk] Disconnecting cables and lightning (questions --

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Subject: [TowerTalk] Disconnecting cables and lightning (questions --
From: (Tom Rauch)
Date: Fri, 05 Jun 1998 09:43:31 +0000
To: <>
> Date:          Thu, 04 Jun 1998 15:51:17 -0400
> From:          Bill Coleman AA4LR <>

Hi Bill,

> Well, I'm not a lightning expert,

There might very well be no such expert, because lightning 
is so esoteric. Roy Lewallen would call this a "high-Q" thread, 
because it could ring on and on. Wes Stewert calls it a tar-baby. I 
think this is a high-Q tar baby. ;-)  There are some things that are 
not debatable, however, because they can be confirmed by 
repeatable experiment. 

> A grounded conductor aloft assumes the same potential as the ground it is 
> connected to. It therefore becomes difficult for it to build up a charge 
> and make it a potential target.

Actually not according to physics. Charges DO move up in the 
conductor. The problem is we can't really measure those charges by 
normal means we are accustomed to in conventional circuits, and they 
move up in the conductor even if the conductor is an insulator!!!

Earlier I gave the example of a charged paper cone. That's a common 
physics experiment. That charges all collect on the outside of the 
cone. If you use a string to turn the paper cone inside out, the 
charges all move to the outside again even though the cone is an 

> An ungrounded conductor aloft assumes whatever potential it wants. If it 
> is bombarded with rain, it picks up the charge of the rain. Wind friction 
> can also generate a sizable static charge. It may pick up enough of a 
> charge to become a target.

Here's how I view the problem, I hope it makes sense. Imagine the 
great potential difference between the earth and cloud required to 
start an arc. It certainly is much more potential than a few dry 
cells, hi hi.

Now how on earth (pun intended) can that conductor "collect" enough 
charge to arc over to a cloud thousands of feet away without arcing 
over to earth LONG before that point? The charges physicists speak of 
"collecting" do so because of the force on charges due to the 
electric field, not because of something  like water into a bucket. 
We can't "pour" those charges anywhere because more charges will 
quickly fill the void EVEN when we talk about charge in an insulator!
> Of course, lightning is dumb, and it doesn't understand theory - it 
> strikes where and when it wants....

Things can sometimes be too complex for a simple "one-fits-all" 
explanation. You can bet there are a lot of things going on that 
control getting a hit or not, but changing the potential of a 
conductor tower with reference to earth would make no difference at 
all. Draw it on paper. Lets say the cloud is charged to +5000 volts 
with reference to earth. A conductor connected to earth would 
have a potential difference of 5000 volts. If we let that tower float 
to 2500 volts, the potential difference to the cloud and to earth 
would EACH be 2500 volts, for a total path potential difference of 
5000 volts.

The only way to lessen the charge is to transport charges BACK to 
the cloud that were given up, and that requires some direct path for 
ions or charges. 

1.) I don't disagee that adding a static brush MIGHT reduce the 
potential for a hit, because the corona from the brush would make the 
object appear fatter and blunter. But the very same thing could be 
accomplished by making the object fatter and blunter WITHOUT the 
noise generated by corona. 

2.) I don't disagree a wider copper flashing has less inductance than 
a narrower "something else", but I disagree with the contention that 
the flat shape is always better because it is flat. A one inch 
wide flat conductor has about the same inductance as one inch 
diameter conductor of the same length.  I don't rush out to buy 
copper flashing UNLESS I need a whole lot of area. Three inch wide 
copper flashing is better than a 3/4 inch copper pipe, but I simply 
elected NOT to mess with three inch wide copper flashing running all 
around my house because I can disconnect the antennas when storms 

I am curious about determining some "rule of thumb" for the performance 
of various lengths and materials for broadband grounds. 

This morning there was a loud boom. Marsha was standing in the 
carport watching the storm, and tells me lightning hit directly on my 
200 foot tower. This PC was also on and running. The only damage was 
to a digital wall clock sitting on the refrigerator and plugged into 
a separate outlet. SK clock.

Does that prove anything? Not a thing except nothing is greatly 
wrong with my system. One or two data points mean nothing in a random 
event. Certainly everything survived without expensive gizmo's with 
what I consider proper grounding, but who knows about the next time.

With another  200-300 foot tower going up on the highest point for 
several miles, I'll probably enjoy that next time soon enough.

73, Tom W8JI

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