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.
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. 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 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.
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
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!
Grounding
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 feedlines.
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 power lines.
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.
Summary
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. Antenna Technologies Limited Company 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.
References:
1. “The “Grounds” for Lightning and EMP Protection”, by Roger Block, published by PolyPhaser Corporation.
Antenna Technologies Limited Company and the author retain the rights to all intellectual property contained herein.
This information should be used as a guideline only to help you in the appropriate selection of an antenna.