Shielding for Lightning Magnetic Fields

Lightning Protection Information


How to minimize lightning-produced magnetic fields in the equipment building.

Shielding for Lightning Magnetic Fields

Lightning’ s high current means that the associated magnetic fields will radiate and cross couple energy to wire runs inside the equipment hut. Too often sites are designed to have a good 5 ohm ground system, but the building is placed right next to the tower with little or no magnetic shielding. The distance between the tower and the building is usually kept small so that the transmission lines are short and have minimal loss. Closer tower spacing increases the current the building’ s perimeter ground system must conduct to earth. It also allows stronger magnetic fields to transverse the building.

Aluminum buildings do little to attenuate this magnetic flux. Concrete with steel mesh or rebar, which is ferrous, will attenuate only some of the field. Steel shipping containers, either single or double walled, will act as a Faraday shield for both radiated (plane wave) RF energy and for magnetic (H) fields. The containers will also provide a good uniform ground for your equipment from anywhere inside.

The only other alternative is to use distance (one over the distance squared) to attenuate the strike’ s powerful H field from getting right to the circuit board or chip, causing upset or damage. Distance will also add length to your transmission lines adding desirable inductance, and allowing additional time for the tower ground system to handle the tower surge current.

The inductance benefit of the additional transmission line in the path to the hut will force more surge current down the last bit of tower, from the last grounding kit to the tower ground, instead of over the coax lines to the building. This distance based inductance is not the same as adding inductance by making drip loops with your coax. These loops can couple more energy than they limit. The magnetic fields can be captured and "multiplied" by the alignment and turns ratio of the loop(s). Please note that a straight run from the tower is orthogonal to both the magnetic and electric fields and cannot capture any additional energy.

Appropriate application of inside equipment room "halos" can also reduce the magnetic field. (See "Halo" topic.)