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PolyPhaser: Coaxial Cable Lightning Protectors

Coaxial Cable Lightning Protectors

Antenna manufacturers utilize shunt-fed dc grounded antennas as a means of impedance matching and providing some form of lightning protection to their customers. It has been proven that these antennas do work and should be used as a means of diverting a portion of the direct strike energy to the tower and its ground system. Unfortunately this protection is designed to help the antenna survive and not the equipment. A direct hit, or even a near hit, can "ring" an antenna whether it is grounded or not since it is a tuned (resonant) circuit. The ringing waveform will contain all resonances that are present in the antenna and its coax phasing lines. This means both "on frequency" ringing and other frequencies present will be propagating down the transmission line towards the equipment. The "on frequency" energy will not be attenuated by a high Q duplex filter or a 1/4 wave grounded stub being used as a protector. In both instances, the "on frequency" energy will pass right through. Also, if we look at a typical dc grounded/shunt-fed antenna at the top of our 150-foot tower example, both the center conductor and shield will be at the same 243kV potential above ground at the antenna feed. Although the grounded antenna will help prevent arc over of the transmission line, it will have a 6kA peak current traversing its length. The same parallel tower segment will have 12kA. The shared strike current, between the tower and the coax, will contain mostly low frequency components.

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