[Elecraft] Lightening damage

[K9MA]

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

-- 
Scott  K9MA

k9ma at sdellington.us