Part of the reason for the arrestor on the coax line is because the center
conductor is going to have a voltage different from the shield.
The tower and all of the lines coming down the tower share any strike
current. Even if the center conductor is bonded to the shield at the top of
the tower you will still end up with energy on the center conductor.
Because of the high current on the shield during a strike, the resistance
of the shield allows voltage drop over the distance of the cable. Energy is
induced onto the center conductor.
Because the propagation time on the inside of the cable is slower than on
the outside you end up with a voltage difference at the end of the cable
between center and shield. For more information on this look up "transfer
impedance".
This center conductor voltage can be induced on the cable even if the cable
is on the ground. Anywhere there is current on the shield some of the energy
ends up on the center conductor.
This is why you want the protector close to the shack. The length between
the tower and the shack can allow the energy to be induced onto the center
conductor.
This current can come from the tower strike or ground currents from another
direction.
One other important thing that no one has mentioned, although most here
know, is that your AC power must also be protected and bonded to your
entrance panel where the coax suppressors are installed.
You do not want any difference in potential between the AC power and your
coax lines. The easiest way to accomplish this if your power panel is not
close to your coax suppressor panel is to run an AC line (large extension
cord is fine) over to the coax entrance panel. Install AC protectors on that
line with them bonded to the coax panel.
Feed all of your power and antennas from only that panel. This will insure
that everything stays at the same potential. Of course include any phone
lines, rotor cables etc the same way.
73
Gary K4FMX
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