ProSoft Technology ILX34-AENWG - Manual (Page 76)

PSW-PWD ♦ ProSoft Software Lightning Protection
ProSoft Wireless Designer User Manual
Page 76 of 91
ProSoft Technology, Inc.
July 8, 2009
9.2 The Nature of Lightning
Lightning can form and stay in the upper atmosphere. This is often a beautiful
sight to behold as the bolts jump from cloud to cloud. On the other hand, when
lightning leaves the clouds and strikes the ground or a tall object, it can inflict
instant destruction and even death to those unfortunate to be near the strike.
The energy present in a lightning bolt can be considerable and a direct hit will
inflict the maximum damage. It is estimated that a typical bolt may contain a
potential of millions (1,000,000’s) of volts thus generating currents up to 100,000
amperes! That is very destructive energy. At the same time, the heat in the bolt
can have a temperature up to 30,000 K, hot enough to start fires.
A lightning bolt will often drag or jump along the ground. Therefore, there can be
considerable damage in a wide area (even hundred’s of feet) surrounding the
strike. Just ask a dairy farmer what happened to his cows that tried to hide
beneath a tree that had a direct lightning strike.
9.3 Lightning Protection in General
Lightning protection must be examined from four distinct directions. First off, the
place where the antenna is mounted (such as on a tower) is important. Then
there must be input protection from the lightning strike itself, typically in the form
of a huge and rapid build up of voltage and current at the input to the radio.
Thirdly, a proper ground system must be employed to rapidly conduct the
lightning bolt energy away from the radio. Finally, protection is required at the
output or main power supply such as the line voltage supply (for example, the
115 VAC we obtain from a line cord). Let’s discuss each of them separately.
9.4 Antenna Mounting
It is well known that lightning statistically strikes the highest electrical conductor
in an area and then follows the lowest resistance and shortest path to ground.
Because antennas are usually mounted in high places, they are very susceptible
to lightning strikes. Most antennas have a metallic boom and the elements are
often attached directly to the boom so they are a likely target for a lightning strike.
Therefore, the antenna location and how it is mounted is probably the most
controversial topic when any discussion of lightning occurs. Ben Franklin gave
rise to the theory that the lightning was electricity and found this out when he
almost was killed by a lightning strike conducted down the wire holding down his
kite. Franklin is sometimes credited with the ball discharger and the pointed rods
on houses. As a result, to this day lightning rods with grounding wires are a part
of folk law and many are installed on high buildings and homes, especially in
areas prone to lightning activity. Properly installed and grounded, these devices
surely do work.