CBWI February 1997 MODERN LIGHTNING PROTECTION FOR RADIO FACILITIES: RF ENTRY PORTS
Lightning is one of Nature’s most destructive forces. It has the power of a good sized explosive and cannot be avoided if you’re connected to antennas that are high and in the clear. And it’s not just lightning. On a recent evening our 160 Meter dipole (260 foot wire span) strung between towers at 180′ here at the I.C.E. Factory exhibited several hundred volts of charge from a light rain shower – enough to shock one of the technicians working with the cable outside. During an electrical storm with overhead discharges many thousands of volts have been measured on this wire, respective to earth.
In installations using coaxial feedlines the measures used to protect station equipment are simple but critically important. Here is a list of observations and our recommendations in the strongest possible terms …..
1) Always bring coaxial cables to ground level before entering equipment area. Never bring coaxial cables into the building at an elevated height directly. Lightning currents induced into the cables will be forced throughout the equipment chassis on their way to ground, and that’s what causes extensive damage. Even if your equipment is on the second floor, always bring coax to ground level first and insert appropriate lightning protection, then route the cable to the station.
2) Absolutely, absolutely, positively, positively ground those shields with as short an earth terminal connection as possible. Use a commercial shield grounding block is possible, or fashion your own. In most cases as much as 80% of an induced or direct lightning blast comes in on the shield. This is because of the external exposed nature of the shield and its larger metallic mass. Always make sure that grounding the shields occurs BEFORE the cable enters the building. Multiple shield grounding (such as once at the tower base and again before building entry) is an excellent idea.
3) Use lightning arrestors on lines that feed sensitive electronics. But beware. Don’t use so-called lightning arrestors that employ nothing more than a gas-discharge device to ground. These units are DC passive and only activate when the potential voltage between conductors reaches hundreds of volts. By that time in most cases the radio has already been damaged before the arrestor kicks in, leaving you with an arrestor that did mostly nothing and a damaged rig. Additionally, gas discharge tubes are very low power, typically only around 1 watt dissipation. They’re rated for 20,000 amps or more, but only if a lightning blasts starts and ends in a few billionths of a second. Few bolts ever do, and bolts that are slowed down coming through transmission lines almost never do. That’s why gas discharge arrestors require repair and replacement so often. They’re overpriced and offer little, if any, protection from induced voltages. If lightning. arrestors are used always specify a blocking type arrestor – that is, a unit that has no DC continuity through from input port to output port. And one that offers constant drain mechanism with no pre-determined turn-on voltage has enormous power handling capacity, far exceeding the units that rely solely on gas discharge tubes or varistor devices.
4) Establish a grounding. bulkhead near the radio equipment where the distance from the bulkhead to the soil entry is short – preferably less than a, foot. Use this bulkhead for lightning protection as well as RF neutral for interference filters and similar items. The bulkhead can be a bar, metal sheet, or just heavy wire. Remember – the length of ground leads is far more significant to good grounding performance than the specific materials or even wire size used. Keep ’em short! ©CBWI