Folks,
There's an aspect of this that hasn't been touched on,
that has to be considered.
Let's assume that you have a really good dissipative
ground system from said tower. Multiple rods, multiple
radials, smooth strap transitions from tower to gnd, etc.
And let's further assume that you run 4" strap around the
perimeter to the AC service entrance ground.
So...you've about done what you can.
Lightning hits the tower. (or a nearby tree, even).
Current flows to ground.
Some distance away, within the house, you have a
(telephone(stereo)(TV)(Ham gear).
It's grounded to the power system ground. It also has
wires dangling off it, for purposes of signal or data
reception.
Consider that you now have a transformer.
The one-turn primary is the antenna/tower/ground system. (or tree, or water
tower)
The one-turn secondary is the tv cable, tv set, power ground path.
Or whatever equivalent you have in the shack.
I have measured 1500V between a 10m ground plane and earth ground with a
dmm,
when lighting struck a water tower, 300 yds away. It was struck 7 or 8
times
in one storm, alone. Imagine what might happen if the distance were much
shorter?
So...you can take all possible precautions with SPG and good grounding
techniques,
and still be done in by induced currents in the "transformer secondary"
paths.
This is why I disconnect my station completely from antenna, power and
network,
during storms. Incidentally, if you leave the radio connected to power and
RF GND,
whatever potentials exist between the RF gnd and the power gnd are impressed
across the ground traces in the radio...and power and RF are undoubtedly
kept separate
for EMI/EMC/noise reduction purposes.
Just a thought.
n2ea
jimjarvis@ieee.org
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See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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