I have been following the threads about lightning with great interest,
starting from way back in the past archives of this reflector. There are
some really great comments in the archives from people all over the world.
I live in an area that has frequent thunderstorms and have always been
both fascinated with and scared of lightning. It started of course, as
terror when I was a youngster and my older brothers and sisters would
unscrew fuses in the circuit box to make the lights go out after large
thunderclaps just to hear me scream! ha ha ha. They thought it was funny,
but I was scared witless! Fascination part has replaced terror as I have
grown older. Now that I am planning my first tower, my interest has been
intensified and focused.
I have been compiling a document full of the comments on this subject from
the archives. It's very interesting reading. Combining this with other
sources of information (books, TV programs about lightning research,
amazing photographs of lightning leaders, etc) I have noticed a pattern of
understanding that seems to fit in well with just about everything I have
learned.
Charges build up in the clouds due to movement and turbulence. These
charges attract opposite charges on the ground. When the charge and the
associated voltage potential grows large enough to overcome the air gap
between ground and cloud...look out! The cloud may encounter a mountain or
other very tall structure, whereupon the ground charge climbs it and the
air gap is suddenly reduced. Streamers coming down from the cloud meet
leaders that jump of of objects on the ground, and a circuit of ionized
(and now conductive) air completes the circuit.
This is a highly variable process. The cloud floats along above,
dragging the opposing ground-based charge across varied terrain. Objects
on the ground under the cloud become charged with high voltage. Objects
with sharp edges or points will tend to launch their charge into the air,
ionizing it and dissipating the ground charge and reducing the voltage. If
the voltage is reduced enough, a leader and strike won't form.
Conductive objects can carry charge away from the ground faster and can
reduce the voltage potential. It is somewhat self-regulating. In the
absence of objects that can *collect* and *dissipate* the charge into the
air, the voltage will build to such a level that leaders will form from
any object because of the sheer high voltage accumulated in the cloud and
ground underneath. The size, moisture content, shape, etc of the cloud and
terrain determine how much total charge is stored and therefore the
severity of the strike in terms of total current.
Where do lightning rods come in? They help disspiate the ground charges
at the lowest possible voltages with their sharp geometry and highly
conductive path to ground. Not completely, but enough to prevent a leader
from forming. No leader, no strike. On old farms and barns you frequently
see an array of them, each with a ground rod, providing multiple
ionization points to collect and dissipate more charge from the earth.
However, if the ground charge and voltage are high enough, due to an
unusually severe storm, perhaps, a leader will form from the tip of one or
more rods. The one that connects first takes the bulk of the resultant
strike energy. In this case, the lightning rod and its ground system lead
the energy in the safest, shortest path to ground, around you and your
family!
A tower acts like a lightning rod. The more sharp edges on it, the sooner
it will begin bleeding off ground charges into the air. If your antennas
have edges (most do) they will help disspate charge too, resulting in the
corona noise. If your vertical folded dipole array for your repeater is DC
grounded, its mast will perform the lightning rod's function. If you
provide other points, such as the 60 degree points on lightning rods, the
bulk of the ionization will not form on your antenna.
Discharge units ("porcupines") promote the dissipation of charge. But the
fine wires can be melted by the strike current. The more effective your
ground system is, the better your tower will bleed the ground charges, and
a higher percentage of strikes will be avoided. The cloud will move past
your location until the terrain is more favorable for high voltage leader
formation, and the strike will occur elsewhere.
However, eventually, an irresistable charge will come along and you will
have a strike. You have to provide a direct path to a "good" ground to
shunt the current. This is the second purpose of good grounding.
Ohm's law comes into play. Huge current through any resistance creates a
voltage drop, and you get "side-flash" from objects that are not at the
same potential. Even if the main strike doesn't hit your tower, you can
still have powerful streamers and short leaders from it that didn't
connect, yet still produced damaging current and voltage.
Then, of course, you have EMP. It's like a gargantuan, jigawatt broadband
transmitter (literally a spark gap)! A conductor of any length, especially
in the near field (like golf clubs, utility lines, lawn chairs,
umbrellas, etc) whether or not it is struck directly, will resonate with
some wavelength in the strike and become an instantaneous source of high
rf voltage. YOWIE! You can have of mini lightning bolts ("side flash")
jumping off of these objects even though they are not even in the original
strike path. SURPRISE!
Ok, this is getting long. Let me sum up. You need a combination of
measures to mitigate the multiple ways that lightning can ruin your day.
1) It starts with excellent grounding via multiple ground rods tied
together to collect charge current from the ground.
2) Then you need high-mounted discharge points to prevent leaders,
preferably above other objects to be protected. This prevents strikes from
forming as much as possible.
3) Failing prevention, you need to provide a preferred, continuous, low
impedance strike path to ground (tower legs then copper strap to tallest
object on tower such as lightning rod or mast) to shunt the current back
down and distribute it effectively, starting at the highest point on the
tower.
4) Bond metal objects near the strike path together with straps to prevent
voltage potentials generated by the huge strike current. Bond shields and
arrestorst on wiring to the tower at the top and bottom.
5) Use lightning arrestors on wiring/coax anywhere near the strike path,
and on any wires in the near field connected to sensitive equipment.
6) Break as many conductive paths as possible from the strike path (tower)
and you, either by disconnecting and or shunting in a bulkhead. Floating
conductors are still subject to EMP and can still shock you.
I hope some of you find this soliloquy helpful or at least entertaining!
Hi Hi!
--...MARK_N1LO...--
--
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