Ron Nott is a noted RF Engineer and designer of lightning protection systems.
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Radio World, March 1,
Lightning and static electricity have been costly nuisances to the broadcast industry since it began. Lightning arresters, ball gaps and lightning rods have all been installed with the assumption that, sooner or later, one will have to take a strike, and with the hope that these devices will somehow protect the station's equipment.
The study of atmospheric physics shows that it is not necessary that a station ever be struck by lightning. Proper application of scientific principles can prevent a strike altogether by discharging the high voltage electric field that occurs during thunderstorms and other atmospheric disturbances.
Although enormous voltages develop, often exceeding 100,000 V, they can be discharged below the point where lightning occurs, thereby providing protection for a broadcast or communications facility.
Unfortunately, a large amount of misinformation has accumulated over the years, much of it now accepted as fact by virtue of tradition. What follows is a simplified explanation both of what occurs during atmospheric disturbances and of a method that has been proven to work.
Although conductive objects, such as broadcast and communications towers, disrupt the gradient, normally nothing occurs because the electric field is of such high impedance. The energy levels are not there to cause a large, sudden flow of current, as when lightning occurs.
On the other hand, storm conditions cause an inversion of the polarity. The earth, relative to the storm cloud bottom, becomes positive. An enormous, high energy field of many megavolts develops between cloud bottom (negative) and earth (positive).
Many factors affect this field. Wind, rain, the terrain, trees and manmade structures all have an effect, thus distorting the field and causing it to be more concentrated in some areas and less in others.
Causes a Lightning Strike?
The shape of the points across which the high voltage is impressed has a definite effect on the voltage at which conduction by ionization begins.
It has long been known that rounded shapes increase the ionization voltage. When arcing is to be avoided, such as on high voltage transformers and RF components, great pains are taken to avoid sharp points and corners. Surfaces are polished and/or covered with insulation material. The electric charge is distributed over the rounded surfaces, and thus the air surrounding them requires much greater voltage to ionize.
The ultimate shape is apparently spherical. When high voltage experiments were in vogue about a century ago, the highest, most spectacular voltage discharges were between large round balls. With other shapes, particularly sharp points, ionization occurs at much lower voltages, often in the form of a continuous current of very small value, discharging the voltage between the electrodes.
by Low-Voltage Ionization
Blunt-tipped lightning rods are intended to intercept rather than to prevent lightning strikes. They are intentionally located at the point of maximum dielectric stress. (See Figure 2).
While they may protect VHF and UHF antennas by diverting the lightning strike, blunt-tipped rods cause a large current pulse in the tower structure and its attachments. Welding of lighting conduit and coax hangers may occur. The pulse may be coupled into transmission lines, traveling into equipment and possible damaging it or causing a tripoff.
The single sharp-tipped rod precipitates low voltage ionization, but is limited to only a small amount of discharge current. It is analogous to placing a single resistor of large value across the terminals of an enormous capacitor. With time, it could do the job, provided that no additional charge were added to the capacitor.
Unfortunately, this is not the case with a storm. Formation of a storm cloud is a dynamic process, with its charge building at a rapid rate. While a single, sharp point does have a small discharge capacity, it takes many sharp points to discharge the energy of a potentially catastrophic storm.
Air ionization around a sharp point begins to occur at about 10-kV. This may sound like high voltage, but it is minuscule when compared with the millions of volts normally required for a lightning strike to occur.
While it is impossible to discharge all the energy in a storm, induced ionization can reduce the E-field in the vicinity of a tall structure to valued far below those required for lightning to strike, provided that enough sharp points are present in the discharge area.
A high energy storm would require a larger number of points to keep the voltage gradient reduced below the point where lightning may strike. The goal of any dissipation system is to protect the structure by preventing the voltage differential between it and the storm system from reaching the catastrophic discharge point.
Note in Figure 4 that the iso-E lines curve inward toward the structure below the dissipation device. Experience has shown, particularly with a tall tower, that even though the top is protected by a dissipation device, the voltage gradient can still get high enough to cause a strike on the side of the tower. This has occurred on a tower of only 400'. Before dissipation devices were installed, it received many strikes on top, damaging the transmitter and causing tripoffs. The dissipation devices stopped this, but on at least one occasion, lightning hit the microwave STL antenna located at the mid-point (about 200'). Additional dissipators were furnished to solve this problem.
Depending on the weather history of the area, tall towers may need a system composed of dissipators installed at intervals, or even continuously, up the faces of the tower.
There are several solutions to this problem. Resistors are available that are placed across the insulators to drain off the charge.
Another method goes way back in time. When guy wires were installed using Crosby clips instead of "preforms," some tower installers would extend the guy wire 6-8" beyond the last clip and flare out the strands. In effect, each guy wire had a dissipation device composed of the sharp wire tips at each end.
Once a tower has been installed, it is not convenient to go back and do this, but dissipation devices are available that accomplish the same thing.
Whether the guy is discharged by a resister or low voltage ionization, the end result is the same. The flash over across the insulators is eliminated. Occasionally, guy wires receive direct strikes and have even been partly or completely burned in two. One or more of the above methods can resolve this problem.
Another problem eliminated by dissipation devices is the static electricity charge deposited on AM antennas by snowflakes or wind-blown dust. These particles become charged by their motion through the air but cannot discharge because dry air is such a good insulator. When they strike a metallic object, such as a tower, the charge is transferred to it.
Large voltages can accumulate on an insulated tower, to the point where it can arc across an insulator in the tuner or transmitter. The surge may damage components such as capacitors or may trip the transmitter off.
West Texas is known for its wind-blown dust and dry air. A station having it's transmitter site near El Paso had many annoying tripoffs each day during dust storms. A dissipation device was installed atop each of the four towers in the directional array, thus completely eliminating the tripoffs, as well as providing protection during thunderstorms.
The dissipators are physically small and have no effect on the impedance or directional pattern.
Ionization discharge noise can sometimes be heard on car radios while in the vicinity of a storm or atmospheric disturbance. Depending on the size and shape of the corona ball, the ionization voltage can be moderately high. The sound from the radio may sound like the buzz from a relaxation oscillator, sometimes varying in pitch, depending on the storm intensity. Therefore, it is not appropriate to install dissipation devices directly on or in the very near field of receiving antennas. The devices should instead be mounted on the supporting structure, preferably above a receiving antenna.
During atmospheric disturbances, the continuous, low-voltage discharge will generate a low level electrical noise. However, if the signal-to-noise ratio is high enough, the effects should not be apparent in a receiver. If the static charge is not dissipated by low voltage ionization, sudden bursts of noise may be heard in the audio. This is the result of ionization occurring at higher voltage, which causes electrical noise of much greater energy levels.
Received signal quality and reliability can therefore be improved by utilizing low voltage ionization for static charge dissipation.
There is no apparent effect on transmitting antennas, since the discharge currents are very small in relation to the normal antenna current. If the devices is mounted directly on an antenna, such as an AM broadcast tower, it must be physically small enough to have no effect on the antenna field or impedance.
Fortunately, devices have been developed which provide effective lightning protection while having an insignificant effect on the AM antenna.
Static charge dissipation by this method has been used for many years by the aircraft industry. Small strands of conductive fibers or metal are located on the trailing edges of wings and other surfaces to neutralize the charge between an airplane and the surrounding atmosphere, preventing lightning and static charge buildup.
Architects often pointed rods around the periphery of buildings in their designs. It is a proven technology which can be utilized by the broadcast and communications industries to protect their facilities.
Design is primarily a function of structure height, history of lightning in the area and degree of protection required. An inexpensive device can reduce the chance of lightning by perhaps 99%. The additional 1% can be very expensive.
Foremost in the design must be ruggedness, because any device of this kind will be exposed to the greatest weather extremes.
Another essential is a good discharge path for the device. There must be good, low resistance DC continuity between the device and earth ground. AM stations already have this in their ground systems, provided that a ground system is properly maintained and a path is provided in the form of a lightning choke or a static drain choke or a static drain choke across the base insulator. However, it is still a good idea to place at least three 8' copper plated ground rods around the base of the tower.
It is important to remember that a static discharge device normally operates with a small DC current, usually only a few milliamperes. A severe storm may cause a flow of a few amperes, so great pains and expenses need not be taken with large copper conductors.
Remember, the methods of static discharge by low voltage ionization may eliminate lightning strikes completely. Lightning strikes are not transferred to other structures; the energy is instead dissipated by a continuous low-current, low-voltage flow of energy between a structure and the surrounding atmosphere.
Instead of an instantaneous zap of a few microseconds duration, a steady flow of electrical energy occurs over many seconds, minutes or even hours. And it's not complicated or expensive.
I would like to give credit to Richard Ives, PhD, physics, and others at San Juan College for their assistance in verifying the technical accuracy of this article.
CAN LIGHTNING STRIKES BE PREVENTED?
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