sharp corners of equipment cabinets can breakdown the air, causing current to flow. This will be a very low current unless another path is found by these streamers. Heater vents and electrical conduits which are not grounded to the single point can become such paths. It is a good idea to bond (ground) all conductive objects within 1 meter of any single point earthed equipment in the room.

Tower mounted equipment is similar to the above high rise situation. The l/O’ s must be protected and the protectors must be located and bonded together. Single point grounding should be easy to do if the equipment is mounted inside a metal enclosure.

Ground mounted vertical antennas require the same type earthing for lightning protection. A vertical antenna’ s impedance is half of a dipole’ s. This means, the better the ground plane the worse the VSWR match to a 50 Ohm cable. A vertical with a poor ground plane may give a better match. As the ground plane is improved, the match will get worse (35 Ohms approx. for a full sized 1/4 wave). Don’ t stop short of a good ground plane. The better the ground plane for RF, the better the earthing for lightning. This is assuming that the RF ground plane is in the ground.

Do not use trees and wooden poles to support antennas. If you are already using one, the best thing is to place two copper straps on the pole/tree. The straps will ground a VHF/UHF antenna and divide the strike currents with the coax cables. By using two straps, the inductance is further reduced. It is best to place the straps on opposite sides of the pole to reduce mutual inductance. The coax line should be run down between and clear of the straps.

If you have a transmatch fed long wire and the pole is just supporting the antenna wire, it would be a good idea to have the grounded straps extend higher to intercept a strike or to divert energy to ground if the wire is struck.

This can be done by either placing a high voltage gas tube between the long wire and the straps or by making an arc gap between the wire and the ground straps. A gas tube will not be adversely affected by temperature, humidity, pollution, wind or other environmentals, while the air gap will be affected. It may be difficult to calculate the voltages present at the gas tube and it will change when switching bands. A rule of thumb is for about 7kV. An air gap would be about 0.175" at sea level with 50% humidity and grows larger with elevation/humidity. (Humid air is less dense)

Another gas tube or gap may be added closer to the match box. For dipole antennas with baluns, use the same gas tube technique. Place gas tubes around the balun. Place one across the balun at the dipole wires and one from each side of the balun to the ground straps. This will protect the balun from a strike to the dipole wires. The more strike energy you can divert to the ground before it reaches your equipment, the better off you and your equipment will be.

Just a word to those who tell us that they are safe from lightning because they always disconnect the coax from their equipment. When asked what they do with the disconnected line(s), they usually respond that it is placed on the floor. Now if you stop and think about the last few thousand feet that the lightning has jumped, you can see the fallacy of their thinking. In fact, they made it worse since arcing involves ignition temperature

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