| 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 maybe even here in the USA (see strike map at the bottom of this page). 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. 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. Always try to keep the lightning and the protection devices outdoors wherever possible! Antenna Tower Grounding The proper Earth grounding of your antenna tower is essential for lightning protection and static discharge. Many towers are inadequately grounded by using only a few grounding rounds and large gauge round copper cables. This is not correct. The small number of grounding rods are inadequate, and round copper cable has a relatively high impedance to an instantaneous rise in electric current (lightning hit). Extremely high voltages will develop across these cables and instead of going to ground, these charges will go directly into your building equipment. A minimum of four ground rods per tower leg with some sort of chemical grounding material should be used. The chemical grounding material will help to lower the ground rod resistance. Copper straps should be used to connect the ground rods to the tower due to their low inductance. In areas with sandy soil and/or excessive wind-generated static, it's advisable to use a more elaborate grounding method. Most likely a radial grounding system like that found in AM radio. You should also try to have all you transmission line runs inside your tower. This will also help shield them from lightning when it hits the tower. Equipment Grounding A very important part of 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 with copper straps. Substituting plumbing, power ground return and other "so called" grounds for a ground rod is 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. © 2000-2006 MY World Page all rights reserved |