[TowerTalk] Lightning. What is it?

[Roger (K8RI) on TT]

I'll try to do this justice, but it is covered fairly well in a number 
of locations on the web with the Polyphaser site being one of the best.

Lightening is a discharge between two moving, charged areas:  Cloud to 
cloud and cloud to ground.

In the case of cloud to ground we have two "moving" charged areas. The 
emphasis is on moving!
Because the charged areas are of opposite polarity lightening is 
mistakenly thought of as a DC event.

They also vary in intensity from a few hundred to hundreds of thousands 
of Amps

1st the area on the ground is as dynamic as that in the clouds. It 
varies is size, shape, speed, intensity, and direction.
This charge is not homogeneous varying from zero at the edges to a 
maximum at the center.  Instead the intensity varies widely in Jouls 
over the entire area so the  tower at or near the center could be at a 
low point, or even close to zero while areas all around it may be at 
much higher potentials leading the lightning to strike much shorter 
objects...

The charge is "in the ground", Towers, trees, buildings, power line, 
virtually any conductive object.  Said object does not even have to be 
elevated, although elevated objects do present a closer target and are 
struck more often.  But think for a minute.  When a lightening bolt 
travels a mile or two, is it really going to have a lot of concern for a 
path a 100 feet shorter.

Lightening bolts are initiated by "feeders" coming down from the clouds 
and up from the ground.  When these feeders connect a lightening stroke 
is initiated.  This initial stroke may be cloud to ground, or ground to 
cloud.  It will be followed by alternating strokes up and down.  The 
rise and fall times determine the RF frequencies which "usually" center 
around 1 MHz..  What the strike hits may have as much to do with where 
the object is located in the ground field as it does the height. OTOH 
The odds favor the taller object getting struck..

The dynamics of a lightening stroke are complicated as is the 
development of the feeders.  Anyone who has seen a lightening bolt 
realizes they do not follow a straight path.  Lightening bolts contain 
tremendous currents.  So much so that the bolt may be self quenching and 
as we see it flicker, it may follow different paths. I've seen large 
towers struck, with the bolt getting off part way down,  The magnetic 
field becomes so strong that the lightening bolt seeks an easier path to 
ground.

Although the dynamics of the bolt may be complicated, they follow simple 
rules of physics and the parts, we as hams are interested in can be 
simplified to useful levels if we don't "over think" what's happening.  
There is no preventing lightening strikes. ALL we can do, is mitigate 
the effects of the lightening strike, direct hits and nearby strikes 
that can induce very high voltages in electrical wires.  Give it a more 
conductive path to ground and use devices to short voltage pulses to 
ground and keep them out of our fragile and expensive equipment.   The 
Franklin system including lightening rods, porcupines, and other devices 
just provide a path to ground that is more attractive to lightening than 
the structure.

Lightening protection systems, often called Franklin systems are named 
after Benjamin Franklin who is given credit for inventing the lightening 
rod although the use of lightening protection systems predates the 
"Franklin Lightening rod", by hundreds, or possibly thousands of years, 
but Ben is given credit for proving lightening is just a form of 
electricity. https://en.wikipedia.org/wiki/Lightning_rod

It's worth repeating:  There are no devices that will prevent lightening 
strikes.  We can only use good installation and grounding practices to 
mitigate damage from lightening strikes.  We can build fairly effective 
systems, but none give 100% protection.  The more effective the system 
the more expensive it become.  Beyond a good system, building a more 
effective one rapidly approaches diminishing returns for the investment.

-- 

73

Roger (K8RI)


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