How To Protect Your Device From Lightning
Most antenna manufacturers have been asked at one time or another whether their product is lightning proof. Unfortunately, this question is a bit simpler than its answer. In this article you will be able to see some ideas regarding the protection of your installation as well as some widely used protection products.
To start the break down of lightning protection, you first want to look at your antenna mountings. Statistics show that lightning tends to strike at the highest electrical conductor and flows through the lowest resistance to the ground. You will find that most antennas have something called a “boom”, or a metallic rod attached to the antenna, which inevitably creates a strong target for a lightning strike. These days, devices such as static dischargers, wicks, and spine balls are added to the antennas and to the tops of towers. This provides the constant discharge, which in turn, decreases any potential for a direct strike.
As we all know, surge events are not just caught by towers or antennas. The BTS sites or Base Transceiver Station sites are vulnerable as well. If lightning strikes within a BTS, it can prompt high-energy electromagnetic fields onto the feeder lines. To set up a properly grounded transmission system, you will always want a minimum of 3 cable grouping points. The first one will need to be placed at the top of the main feeder line. The second grounding point should be placed at the bottom of the tower-mounted section. This point needs to be within three feet of the lateral bend away from any cable trays or towers. To finish your grounding transmission system, place the third point within three feet of your equipment building or the radio cabinet. Keep in mind, additional points must be placed on the main vertical run of transmission line, usually one grounding point for each 200 feet of additional feeder cable.
Do not forget to set up your input protection as well. This type of protection involves a surge or lightning protector at the input of a radio, or the antenna side. You will find that there are several types of surge protection devices out there, the three major types are the spark gap, the quarter-wavelength shorted stub, and the gas discharge tube.
The oldest of the three is the spark gap, consisting of two points spaced closely together and directly across from the transmission line. Once lightning strikes, the voltage present will jump across each point and travel do the ground. The downfall of this option is that spark gaps may not protect against weaker voltage. The second option is a newer development called the gas tube. The gas tube works similarly to the spark gap, one advantage to the gas tube is that it has the possibility to be designed to dependably operate at significantly lower voltages. The gas tube can operate down to 100 Volts for the low power circuits, and can be devised to operate an extreme broadband frequency range with low VSWR up to 2 gHz. Like the spark gap, if the gas tube does get struck by lightning, it will need to be replaced, although it may not warn the user by initiating a short circuit. Preventative maintenance is required.
Thirdly, the quarter wavelength shorting stub, which has become very popular through out that last several years, specifically above 800 MHz where bandwidth is usually narrow. This device involves a tuned ¼ wavelength shorted coaxial type transmission line that is then placed directly across the transmission line.
Although lightning strikes are inevitable, there are plenty of options available that will give you the protection your equipment requires. With the combination of thorough design, appropriate protection devices, good tooling, and of course proper installation you will minimize the risks. Preventative actions must be taken to make sure there is as little loss as possible. Surge protection and proper grounding is a small price to pay compared to the investment of a complete system replacement due to a lightning strike.