I had a direct lightning strike this summer in July. It has also caused
fire that created more damage then lightning itself. The interesting
thing is that lightning hit not the 20 m tall grounded telescopic mast
that had no antenna (it was down for service), but rather metal roof of
a shed that was only 2 meters high and 1 meter away from the tower. The
the mast was up for 20 years and never before had been hit, although
there were many lightning strikes hitting the neighborhood.
73, Igor UA9CDC
07.11.2017 7:08, K9MA пишет:
I agree that disconnecting the coax from the antenna inside the house
is a bad idea. All my cables from antennas are permanently connected
to a well-grounded and bonded panel in the shack. I disconnect the
coax and control cables from that panel to the radio. Now, if the
tower takes a direct hit, that panel, the ground system for the
station, and all the equipment may go to many thousands of volts above
earth ground, because even my 13 ground rods don't have all that low
an impedance. However, everything in the station should stay close to
the same potential. And, by disconnecting the equipment from
everything except ground, there isn't a direct path for current to
flow THROUGH the equipment.
The hazard, of course, is that there could be an arc from that panel
or the station "ground" through the floor or wall of the basement to
earth ground. That's why it would be better to have that disconnect
panel outside by the tower 20 feet from the house. However, having to
slog through the snow at 20 below in January would discourage
operating. (And we do sometimes have lightning in January in
Wisconsin!) It's a trade-off I've chosen to make. I suppose I could
put the antenna relays, etc. in a box outside, and disconnect it there
in the summer, and have a second disconnect inside for winter. At
this point, however, that outside box would have to get past the
aesthetics committee, and she's none too happy with the tower in the
first place.
About bonding: Lightning is not DC, and the inductance in any
conductor more than a meter or so long can be significant. All you
can do is to try to minimize the bonding inductance by using short,
wide conductors.
In the 28 years my tower has been up, it has not yet taken a direct
hit, though it could in the next thunderstorm. I've had no damage
from nearby strikes.
I think lightning IS more likely to strike a tall tower than a short
bush. Using the "cone of protection" concept, I'd expect the
probability of a strike to go up as the square of the tower height. It
seems as if lightning which, if the tower weren't there, would have
hit the ground within the circle at the base of that cone, is likely
to hit the top of the tower instead.
73,
Scott K9MA
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