The theory of lightning protection by ground is that IF (and that is a
super big IF) all the grounds are short and low inductance and good
grounds that all the equipment and wiring will rise together leaving no
voltage difference to ARC. Of course little things like telephone lines
that are grounded back at the central office can object to the voltage
change and fry modems and phones. Then power lines tend to have all
their grounds connected so that stuff connected to the power line
somewhere develops some voltage differences. By that theory, the more
grounds the better and the better it is to have all grounds connected
together by wires. Which leads to extended lightning damage and also
causes cows sometimes to be shocked in milking parlors.
In my opinion, one can't achieve a low enough impedance in a ground
system (short of tying everything together with 18" wide copper strap
and keeping all the affected apparatus within a smaller space than a
normal residence) so its far better to make the filter more complex.
With shunt elements from tower to ground (e.g. multiple ground rods and
continuous guy wires) then a series element (air gap from disconnected
ground and coax) then more grounding (inside ground and power line
ground). A PI filter or attenuator has always been more effective at
rejection than a single shunt element of dubious quality. Its a concept
beyond those who make the NEC rules. It is important in my experience to
keep that series air gap with substantial dimensions. An inch or two
isn't enough. I now keep it at more than a couple feet and direct hits
to my tower do no damage in the house. There was damage when the gap was
small.
73, Jerry, K0CQ
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Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.
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