Lightning energy can get onto the center conductor of the coax cable buried
in the ground the same way that it gets there where it is hung on the tower
when a grounded type antenna feed is used. Currents can be induced onto the
shield of the cable by ground currents radiated from buried power lines or
phone lines in the area or by currents traveling thru the ground from a
lightning strike being dissipated by a ground system. Those shield currents
can be transferred to the center conductor of the coax.
Google "transfer impedance" for an explanation of the process of energy
transfer to the center conductor from shield currents.
Essentially what happens is the skin effect that makes the shield work as a
shield no longer does so at low frequencies and allows current to flow on
the inside of the shield. Also the resistance of the shield allows current
to penetrate the shield and flow on the inside.
This is why it is recommended that the protection devices be at the shack
entrance.
73
Gary K4FMX
> And you've opened up Pandora's box by asking whether you should add a
> second set of Poly's near your shack entrance. Many here on TowerTalk
> will
> say you need this second set on Poly's. I do not have a second set in my
> installation. Poly's (and other arrestors for that matter) are designed
> to
> keep lightning energy off the center conductor of the coax. Once the
> tower-mounted Poly's take care of that energy, it's awfully difficult for
> more energy to get coupled onto the coax center conductors on the
> (buried?)
> run from the tower base to the shack entrance. It might happen that a
> jumbo
> lightning strike to the ground nearby might couple energy onto the shield,
> but I don't see a mechanism for that energy to get onto the center
> conductor
> for a downstream Poly to take care of for you.
>
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