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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