While the incidence of lightning strikes varies, virtually all populated regions have some lightning exposure hazard. Taken over the lifetime of typical tower and antenna systems, there is meaningful risk worldwide, and it makes sense to at least minimally protect facilities outside of “lightning belts”. In areas of intense lightning risk, extreme protection measures are mandatory!
The largest classes of technical structures needing lightning protection in the world today are wireless communications and broadcast towers and their antennas.
In less than 20 years, cellular mobile towers have proliferated on every continent, and are perfect lightning targets! To a lesser extent, AM, FM, and TV towers have also sprouted, sometimes sharing with cell systems. Not only are the towers at lightning risk, but also the cellular, broadcast, and communications antennas mounted on them. At risk too, are the attached cell site equipment, radio transmitters, coaxial cables, and tower light systems.
There are a wide variety of lightning air terminals in the form of lightning dissipators. Sometimes called a dissipator, or static dissipation array, this relatively new and advanced air terminal replaces conventional lightning rods in most applications. It functions as a streamer retarding air terminal.
Static dissipation array generically describes a system using point discharge phenomenon to protect towers and antennas and the area around them from a lightning strike. These function, as the name implies, by dissipating static electrical charge. Among design factors, the radius of the dissipator electrode cross-section is critical because the process which enables dissipation of static ground charge to the atmosphere is related to electric field intensity (and flux density) surrounding the lightning dissipator. Static dissipation arrays provide, in effect, a “low resistance” route for static ground charge to reach the atmosphere, thus preventing a build up of the ground charge to the value necessary to trigger a strike on the protected object.
Since a dissipation system must provide a low resistance path to the atmosphere, it seems logical to provide as many discharge points as reasonably possible. By using a large number of air terminal points one can compensate for any loss of efficiency from a theoretical maximum, and spread the dissipator elements over more of the cross-section area of the tower or antenna structure.
All objects have natural dissipation points. On a tower structure, charge tends to gather at, and dissipate from the tower top, antennas and antenna mounts, and from corners. The most effective way to mount a dissipator in terms of structure, weight, wind loading, cost and aesthetics is to enhance this natural dissipation by supporting the unit from the structure itself at these natural dissipation points. Since most antenna and tower structures are steel, direct attachment provides excellent conductivity. As a practical matter, the array configuration should be tailored to the structure, not vice versa.