By Joseph H. Reisert
Antenna Lightning Protection
Antenna manufacturers are often asked about lightning and whether
their antenna is lightning proof. This is not a simple question
to answer. In this application note we will attempt to describe
some of the statistical properties of lightning strikes and give
some recommendations on how to best protect your installation
from damage due to a lightning strike.
It is important to point out at the start that
lightning protection is primarily a function of
how much time and money you are willing to spend.
Obviously, the more expensive the radio and the
importance of system connectivity, the more robust
your protection should be.
An Introduction to Lightning
Lightning has been around since the beginning of
time. In fact, at this very moment, lightning
is striking somewhere on the earth. In the lower
48 US states and Canada, lightning is most prevalent
in Florida (with an average of 70-100 storm days
per year!) and the Rocky Mountains especially
in the Colorado area (with an average of 70 storm
days per year). In California and the Northern
parts of the USA and Canada the likelihood of
encountering lightning diminishes to about 10-30
storm days per year. Lightning is most common
starting in the spring and ending in the fall
with a large peak in the summer but it can occur
at any time of the year.
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.
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 (e.g. the 115 VAC we obtain from
a line cord). Let's discuss each of them separately.
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. Since 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
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.
Nowadays, new controversy has resulted with the
use of spline balls, static dischargers and wicks
mounted on antennas and the top of towers. These
devices are said to provide a constant discharge
thus decreasing the potential for a direct strike.
Some users claim a diminished amount of direct
hits after installing these devices.
Suffice it is to say, if at all possible, don't
mount your antenna on the highest building or tower.
Place it a few feet lower and hopefully the fickle
lightning bolt, if it generates a direct hit, will
not discharge through the antenna. Furthermore,
the boom or mast should be grounded to the mast
or tower. More on this shortly. Don't forget to
ground guy wires that are used on stabilize towers.
They are just as likely to be hit since they extend
over a wide area around the tower.
Input protection is typically provided by a lightning
or surge protector at the input (or antenna side)
of a radio. There are three major types of lightning
protection devices for the radio input. They
are the spark gap, the gas discharge tube and
the quarter-wavelength (1/4) wavelength shorted
stub. Each method has its pluses and minuses.
The spark gap is the oldest know lightning protection
having probably been invented by Ben Franklin!
Basically it consists of two balls or points closely
spaced and directly across the transmission line.
When a strike occurs, the high voltage present
will jump across the points and be conducted to
ground. RF transmission devices for 50 Ohm systems
such as this have been around since the 1950's.
The shortcoming with this older device is that
it may not protect against a weak or lower voltage
strike. Adjustment at the factory may produce varying
voltage breakdowns. Furthermore, once a strike
is taken, the device may fail or short circuit
so maintenance is required.
A more recently developed input protection device
is the gas tube. It works in a similar manner to
the spark gap but can be designed to operate reliably
at much lower voltages down to 100 Volts for low
power circuits and 250 Volts or higher for higher
power applications. Another advantage is device
is that it can be designed to operate over a very
broadband frequency range with low VSWR up to about
2 gHz. However, it too must be replaced after a
strike although it will not necessarily warn the
user by going to a short circuit so preventative
maintenance is required.
The quarter wavelength shorting stub is becoming
a very popular device, especially above 800 MHz
where system bandwidth is generally narrow. It
consists of a tuned quarter wavelength shorted
coaxial type transmission line that is placed directly
across the transmission line. Simple types have
a narrow bandwidth, typically 10'-. bandwidth,
but are low loss and inexpensive.
The optimum place to locate an input protector
is at the entry point to the building where the
equipment is located. Don't forget to provide a
low impedance ground connection to the protector
as described below. Always try to keep the lightning
and the protection devices outdoors wherever possible!
The most important lightning protection is a good
low impedance Earth/ground connection to the
associated equipment. The Earth ground connection
should be a copper plated rod preferably at least
5-8 feet in length driven into the ground. This
ground rod should be located as close to the
equipment as possible, typically just outside
of a building at the entry point of the antenna
Greater protection can be provided by using additional
ground rods spaced at least 8 feet from and connected
to the original rod. Substituting plumbing, power
ground return and other "so called" grounds
for a ground rod is definitely not recommended.
Finally, there should be a large diameter (#4
AWG or larger) copper wire connecting the equipment
to the Earth ground. The shorter the wire, the
better. Additional information on grounding can
be found in Reference 1.
Mains and Power Supply Protection
Just because your radio and antenna are miles away
from a lightning strike does not mean that you
are protected. Lightning often strikes power
lines and produces a large voltage surge or spike
that can be transmitted for miles on the main
Therefore, for maximum protection, all power line
interfaces should include a transient voltage surge
protector. These devices are becoming quite common
and inexpensive. Again, there are simple protectors
and those that may include additional protection
with built-in line inductors. Just make sure that
the surge protector is placed between the power
lines and the equipment power supply.
As stated at the beginning of this paper, lightning
damage can be extensive and costly. We have tried
to provide you with an overview of things to
consider when installing a radio and antenna
system but only you can determine how much protection
is required. Astron Wireless Technologies, Inc.
has access to various lightning protection solutions
for most applications. Please call for additional
information on lightning protection devices and
how they can help protect your investment.
1. "The "Grounds" for Lightning
and EMP Protection", by Roger Block, published
by PolyPhaser Corporation.
Wireless Technologies, Inc. and the author retain the rights
to all intellectual
This information should be used as a guideline
only to help you in the appropriate selection of an antenna.
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