[TowerTalk] Where to put Lightning Arresters

[David Robbins]

<The shields of the coax on the tower are bonded and hit the tower's ground
system but the center of each coax is left floating.  My thinking is that I
prefer that suppressor take a hit and try to dump some of that HV on the
center of the coax to the tower ground as best it can there instead of
lighting up the tower switching and the coax back to the shack.  Suppressors
are expensive but compared to the other stuff relatively easy to replace in
the event they die the lightning inspired death out at the tower.>	

Actually... the more common problem is that the tower itself gets hit or a
grounded part of the antenna is hit, or neither is hit and a high voltage is
induced on the shield by a nearby stroke.  That puts the high voltage on the
shield of the coax and tower structure and the center conductor remains at a
relatively low voltage.  Great you may think, all the current will drain off
to ground and everything is happy... guess again, that current will split
when it gets to a point where the shield is bonded to the tower or to an
entrance panel ground with no arrester, but it doesn't just go away, some of
it goes both directions.  So when it gets into your shack there is still a
very high voltage on the shield of the coax when it gets to your radio and
energizes the radio ground... now it has 2 or more paths out of the radio,
one through the power supply to the AC mains, and another through the
receiver to the center conductor of the coax that is still at it's nominal
low voltage.  The arcs caused by the current on the ground going to another
conductor like that are called backflashovers.  What is the fix??  At the
entrance to the shack you have lightning arresters that take some of that
high voltage from the shield and put it on the AC line and coax center
conductor so there is no voltage difference when it gets to the radio.  No
voltage difference, no arcing to let out the magic smoke and everyone is
happy.


David Robbins K1TTT
e-mail: mailto:k1ttt at arrl.net
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