[geeks] Lightning question

[wa2egp at att.net]

> Oh well, here we go again. This is always a hot topic and even the "experts"
> get it wrong. Lightening flows from the sky to the ground. This has been
> proved experimentally, photographically, and logically. 
> 
> BUT the polarity is wrong. Electricity will only flow from one direction
> to the other and the sky is the to, not the from. Then how does lightening
> happen? Air is an insulator of electricity. Lightening can not flow across
> air. The distance is too far for a spark to jump it, like static electricity
> from your finger to a doorknob.
> 
> What happens is that a cloud of ions forms both in the sky and oppositely
> charged ions forms on the ground. This is like the earth being a carpet
> and the clouds being your foot.

The field strength of the charge ionizes the air.  Usually it separates
charge within the cloud.  The charge on the ground is induced by the charge
in the bottom of the cloud.

> >From a point source, the clouds of ions touch and a small current flows
> from the ground up. Once the path opens, a large current flows from the
> sky down.
> 
> So yes, while a lightening rod does not attract lightening as Ben Franklin
> thought, it causes it. The effect is the same, the lightening strikes the
> rod instead of somewhere else.

No it doesn't!  The sharp point on the top dissipates the charge because the
field strength at the point is very high and ionizes the air.  This 
ionized air carries the charge away from the building making it less likely
to be struck. Lightning tends to hit the highest and most charged charged 
object. It's like putting "The Club" on your car.  It still can get
stolen but it is easier for the thief to steal someone else's...

> One way to prevent lightening from striking is to discharge the ions
> before they are concentrated enough to make lightening happen. If you
> look at antenna towers, you often see along them large balls of
> something that looks like a porcupine or steel wool. These dissipate the
> ions and reduce the effect of lightening. They are not 100% effective as
> sometimes just too many ions build up too quickly.

They work just like lightning rods.
 
> In your case, they may prevent lightening from striking on your home and
> due to the fact that electricity takes the path of least resistance, find
> ion paths nearby instead of your home.
> 
> When I lived in the states, I had lightening problems until I put up an
> antenna, a Cushcraft 2 meter (144mHz) twist beam. It was two antennas,
> one horizontal and one vertical, on the same mast, about 11 feet long,
> with 13 elements each about a foot apart. 
> 
> At 144mHz it was a high gain, directional antenna, at DC, which is what
> lightening is, it was a large grounded "ball". 

More of a spiky thing.  Lots of sharp edges.  A ball would prevent 
discharge which is why some high powered mobile antennas have them at the 
end. The corona discharge is a plasma that can actually chew up the 
end of the antenna and on lower end of the HF band its usually the worst.

> After I installed it, I had no further lightening problems. I have no
> idea if they came back after we moved and took it down, but I expect
> the new owners and the people next door were unpleasantly surprised.
> 
> So my advice to you is to get a ham license, and put up a big beam
> antenna that has a DC ground. :-)
> 
> Geoff.

And listen to all the corona noise from the discharge.  Neat to 
hear it build up until a nearby strike discharges everything and
it stops, then build up again.  Works best in AM mode.

Bob