Lightning has a large magnetic field associated with its typical 18,000 ampere pulse. The magnetic field will couple to all nearby conductive materials. There are two ways to minimize the amount of magnetic energy coupling, shield your equipment or place some distance between the equipment and the likely strike location.

A galvanized steel sheet may be used as a shield to attenuate the magnetic field pulse by 10dB The steel should be at least 30 gauge (0.016 inch) and should be connected to the ground system.

Distance is the other means to limit the magnetic field coupling. The strength of a magnetic field diminishes at the rate of one over the distance squared. Since a moderately high tower is much more likely to be struck than any other nearby structure, the placement of the tower with respect to your equipment deserves significant consideration. Factors that should be considered are not only the magnetic energy which will radiate from the tower, but also the benefit of the distance in terms of the inductive loss provided by the length of the orthogonally run coax. This added inductance of the coax line will buffer the energy entering your equipment area. In addition, the extra distance will provide a little more time for the tower ground system to dissipate the strike energy and thus have less to share with your equipment.

Both of these factors indicate there should be a reasonable separation between the tower and the operating equipment. Distances of greater than 20 feet approach reasonable levels. For towers already located closer than this, it may be necessary to utilize some shielding to minimize the magnetically induced energy.

A ground mounted vertical antenna is very similar to a ground mounted tower. Both have a substantial and low impedance connection to the ground system. However, if the antenna or tower is mounted on a roof, the inductance inherent in the conductors to the ground system will be very significant. So significant, that voltages in the order of several hundred thousands volts will be present. To reduce the inductance in the ground conductors, increase the surface area of the conductor (wider copper strap) as well as the number of conductors.

For roof mounted antennas and towers, multiple down conductors can be spread over the roof and brought down to ground in multiple locations. This will require a ground system run completely around the building (a perimeter ground). As an added benefit, this multiple down conductor approach will reduce the mutual coupling between down conductors and provide a low, unsaturated perimeter ground to absorb the conducted surge.

The magnetic fields will also be divided and will, in theory, cancel in the middle of the building. This will help minimize magnetic energy coupling into the wiring inside the building.

Since the tower is a conductor and is well grounded, all of the coax lines should be grounded (using a grounding kit) at the top of the tower close to the antenna and at the base of the tower before they come toward your equipment.

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