The radio engineer does his best to prevent its intrusion into his
equipment, but having a steel structure extending from 150 feet to
1,000 feet in the air invites lightning strikes. Nevertheless, with
proper engineering precautions such structures can survive intense
lightning attacks. The Eiffel Tower, which is nothing more than a
solid-steel lightning conductor, falls in the same category and it
The most important concern in tower construction is complete and
thorough bonding so that there are no areas for excessive heat to build
due to resistance, and no sections that could present high reactance to
the fast-rising lightning strike.
Most people have seen summer lightning or heat lightning; lightning
merely flashing within the clouds often without thunder. The
up-and-down drafts within the clouds in a lightning storm provide the
mechanism that generates the electric charge. This charge forms on the
underside of the clouds, and is called the base charge. This induces a
charge of the opposite sign on the ground.
If the area beneath the storm cloud includes a radio tower, the
ground charge will be concentrated up the tower. When the difference
between the top of the tower and the base of the cloud becomes high
enough, the air dielectric between the two breaks down, a lightning
strike occurs and the cloud is discharged to ground. If the tower is
well grounded and has been properly bonded, no damage will occur to
Lightning dissipaters, such as this
lightning spur, reduce the potential for a lightning strike between the
tower and storm cell by transferring electrical charge to the adjacent
ionizing air molecules, thus reducing the probability of a lightning
strike. Photo courtesy of ERI.
Watching a lightning strike is interesting. Photographs of lightning
strikes have shown leaders that appear just before the discharge
occurs. These leaders seem to develop downwards from the underside of
the cloud in leaps of about 150 feet. They continue to extend toward
the ground until, when several hundred feet from the ground, streamers
begin to rise from the ground toward the leaders. When the leaders and
streamers connect, the ionized path formed provides the path for the
If we could prevent the leaders and streamers from making contact we
might prevent lightning strikes. Aircraft have what are known as wicks
on the trailing edges. These wicks serve the same purpose as the
lightning rods on the top of radio towers. They allow the aircraft to
discharge itself continually as it flies. In the case of the lightning
rod, the sharper the point the better the lightning rod works.
As the diameter of a conductor decreases, the voltage gradient
increases toward the point. When the voltage at the end of the rod is
sufficiently high it will bleed-off some of the induced lightning
charge on the ground, streamers will be reduced or eliminated and the
likelihood of a lightning strike is reduced. A corona develops at the
tip of the rod, which sometimes can be seen.
An ounce of prevention
Several companies offer lightning protection devices that use a
series of sharp-pointed electrodes mounted on the top of the tower.
These are known as static ground charge dissipation systems and can be
effective. It is also common practice to install a static discharge
choke from the base of the tower to ground. However, if the standard
ball gap is not properly adjusted it can cause more harm than good.
AM towers, with their large and low-resistance ground systems, seem
to suffer less damage from lightning. Once a lightning strike enters
the tower, its path toward ground is determined by the reactance in its
path. Because the rise time of the wavefront is rapid a reactance that
might normally be considered insignificant can develop catastrophic
voltages, which can bypass ground paths and jump to adjacent
Different ground points can result in a
voltage differential between devices.
A lightning strike on a power line can enter through the line
itself. Regardless of the amount of lightning activity in a station's
local area, surge protectors are essential. Some power lines contain
intermittent spikes caused by load changes, as well as severe
over-voltage surges caused by lightning. The surge protector should be
located where the line enters the building, and additional surge
protectors on individual pieces of equipment are advisable. It is
essential that surge protectors be connected as closely as possible to
the equipment to be protected, and in no case should long connecting
cables be used.
There should be only one ground system connection. Otherwise it is
possible for high voltages to develop between them in the event of a
heavy strike. The extensive ground system required by AM transmitters
sometimes provides an ideal unity ground. However, all connections to
this ground should be made with a flat, wide copper strap to ensure a
low-impedance connection. If possible, bring all ground connections
close together so that there can be no potential differences between
If a lightning strike occurs near the transmitter building a voltage
gradient will be produced across the area. If these three lines are
brought through widely separated ports, surprisingly large voltages can
develop between them if they are grounded in three places. Locating
them together and connecting them to the same ground should reduce or
eliminate lightning damage.
E-mail Battison at email@example.com.