During the strike event, the tower and the coax lines will mutually share the strike energy. If the coax lines are not grounded as they leave the tower or they are completely isolated from the tower, more energy could traverse the coax toward your equipment than is conducted to the ground system by the tower. Such a large inductive voltage drop may cause arching between the coax lines and the tower which in turn could cause deterioration (pin holes in the coax for moisture to enter) or destruction of the coax lines.

Notice the word "bottom" in this section. Since all towers have some inductance, leaving the tower at a point above ground will allow some of the strike current to continue on the coax line (both the center conductor and shield) toward your equipment. Once at the equipment, the current will follow the chassis to the safety ground.

This could elevate the equipment cabinets to deadly voltages. Deadly for both people and components.

Even though inductive properties of the coax cable appear to be beneficial, and some extra inductance can be created by adding a few turns to the coax; don’ t do it. The added turns can also act like an air wound transformer that can couple more energy into the line. This is just the opposite of the desired effect. Instead, make sure that coax lines leaving the tower remain at right angles to the magnetic field surrounding the tower.

Rotor control lines should be protected using a suitable protector at both the top of the tower where the lines go to the control motor and inside the shack at the single point ground panel.

If it is not practical to protect the lines at the single point ground panel, they may be protected at the bottom of the tower. The protected lines should then be placed within EMT (metal) conduit that is grounded only at the tower-base end. The EMT will act as a faraday shield from the tower’ s magnetic fields and will minimize the amount of induced energy.

Coax lines can also be protected from induced energy by using an EMT conduit grounded only at the tower-base end.

The next step in a good protection scheme is to provide a single point ground, a plate where all of your equipment l/O protectors can be located. The panel is best located near the ground to keep the inductance of the ground conductor low. However, if this requires the plate to be too far from your equipment (more than 10 feet or so) and if the magnetic fields of a nearby tower can easily couple into the interconnecting wires and cables, then the panel should be located close to your equipment.

An alternative to the single point ground plate is to use a rack panel. This is recommended only if all of the l/O protectors are mounted on the panel and the ground connection is directly to the panel and not to any other piece of equipment.

The grounding of the plate or panel is very important. A low impedance path to ground is a necessity and only copper strap should be considered. Since the strap is flat, its susceptibility to magnetic fields is only towards its edges. To prevent coupling, the strap should be oriented with the flat side parallel to the tower (the most 

Previous Page

Next Page