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Portable Wire Antennas

Lightning Protection

         The term "lightning protection" is an exaggeration. It should be called "lightning damage mitigation". It is important to have a concept for the damage a direct lightning strike can inflict. I grew up in the Missouri Ozarks where thunder storms can be intense. Lightning would frequently strike trees and conduct a current through the tree with sufficient energy to flash boil the sap. The abrupt conversion of tree sap to steam would cause the tree to explode violently leaving shredded oak scattered over a large area. An average bolt of lightning would destroy about half of an average sized tree. One day we had an extra large bolt of lightning strike an oak tree in the edge of a field near my grandfather's house. The tree was about four feet in diameter at the base and it was totally destroyed. There was nothing left standing, not even a stump. There was just a hole in the ground with trenches radiating out from it where the roots had exploded out of the ground. The wood was shredded with no single piece more than about two feet in length.

         Now imagine that that lightning bolt had hit your antenna. Even with a proper ground and lightning arrestors, do you think any gear attached to that antenna would survive?

         All metal poles and towers supporting antennas are lightning rods. The antennas mounted on those poles and towers are separate, electrically isolated lightning rods. Lightning rods attract lightning. To keep that lightning out of your house, shack or office, it is necessary to attach a good earth ground to the base of the tower or pole. A good ground means driving several 4-8 ft. metal ground rods into the soil at the base of the antenna support and attaching it to the support with a large grounding strap. The ground rods should be spaced a distance from each other equal to their depth in the ground. The rods should be driven rather than placed in a hole and filled in. Driving the rod produces better earth contact.

         With a coaxial cable, the shield is generally connected to your equipment chassis. The shield can be connected directly to ground at the antenna base providing a direct path to ground without having to go through your equipment to get there. Protecting the center conductor of the antenna coaxial cable is more difficult. You can't connect the center conductor to ground and still operate your gear. There are three classes of protective devices that you can easily purchase and install to partially address this problem. They are:

         So, will good grounding and good protective devices protect your gear from a lightning strike? Maybe! Bear in mind that broadcast stations, cell towers and military stations take lightning hits and mostly survive. Nevertheless, there are horror stories of amateur antennas being hit with lightning resulting in damage or destruction of equipment and building.

         The only way to have your gear protected from lightning is to have your gear disconnected from the antenna and have the antenna cable grounded or removed from the shack. If lightning strikes the antenna, and the bolt has enough energy to blow past the lightning arrestor, it will follow the center connector to the end of the cable and arc to ground. If the end of the cable is indoors, the nearest ground will likely be the ground side of the house electrical wiring. Of course, the arc will blast through anything in its way. For some operators, the standard operating procedure is to disconnect the antenna cable and toss it out the window. Alternatively, you can run a ground strap from your ground into the shack and mount a so-239 connector to it. Then, when not operating, disconnect the antenna cable from your gear and connect it directly to the grounded connector. Some operators connect relays to the feedlines such that the antenna is automatically grounded when the station is not operating.

         When you are operating portable, you won't have a good ground or any ground at all. Common sense requires that you do not put up any antenna when there is risk of lightning. Common sense also requires that you don't leave an antenna deployed when there is any question of the weather changing for the worse. Particular awareness is required when working in the mountains, especially near peaks. The electrical potential between the earth and sky is highest at the peaks, especially when there is some metallic mineral content in the earth. If you ever notice your hair standing up, even a little, or see signs of static electricity, run! Get off the peak as quickly as you can. The high points can be great places to deploy an antenna for DX work, but be extra vigilant regarding the electrical safety concern.

         Another brief digression can be made about using surge suppressors to protect your electronics from voltage spikes on the power lines. I make this point because I have heard telephone company technicians, electricians and others adamantly arguing that these devices do nothing and shouldn't be bothered with. They are flat wrong. The surge suppressor power strips may employ several types of voltage clamping devices including metal oxide varistors, selenium voltage suppressors, silicon avalanche diodes, or thyristor surge protection devices and an electromechanical circuit breaker. The surge suppressors are connected between the power line and ground and present a high resistance at the operating voltage (120 VAC). Voltage spikes on the power line can be caused by lightning strikes, electromagnetic pulses generated indirectly by solar flares, the operation of large industrial equipment or power tools, or by EMP warfare. The voltage spikes coming off the power line can be from several hundred to several thousand volts and usually of short duration, perhaps a few milliseconds.

         The important specifications for the consumer type power line surge suppressors are:

         The circuit breaker is an important component in a transient suppressor power strip. When the transient suppressor reduces its resistance, it presents a short to ground which increases the current through the circuit breaker which trips the breaker and disconnects all of the equipment in the circuit from the power.

         On multiple occasions, I have personally had one of these devices do its duty saving me thousands of dollars in equipment damage.

         When installing grounds and lightning protection components, the details are very important. This short digression from wire antennas is insufficient to address the subject. For more information, consult the following references.

http://ntl.bts.gov/lib/22000/22300/22344/FAA_STD_019D.pdf LIGHTNING AND SURGE PROTECTION, GROUNDING, BONDING AND SHIELDING REQUIREMENTS FOR FACILITIES AND ELECTRONIC EQUIPMENT

http://www.arrl.org/lightning-protection Several articles on lightning and EMP protection and sources of hardware

http://www.lightningsafety.com/nlsi_lhm/lpts.html Fundamentals of Lightning Protection

http://lightning-protection-institute.com/lightning-protect.htm RECOMMENDED GUIDE FOR THE PROTECTION OF EQUIPMENT & PERSONNEL FROM LIGHTNING

http://lightning.org/learn-more/library-of-resources/ Technical Articles on Grounding and Lightning Protection

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