[Premium-Rx] Lightning hitting your Premium RX

[George Georgevits]

Folks,

I've been reading the lightning thread with interest. Danny was the first
one to mention induction from nearby strokes. Most strokes are cloud to
cloud and these, along with nearby strikes to ground, will induce voltage
spikes into your antenna. Your properly grounded coax protector should work
OK on these. There will be many of these induced spikes over time, so you
need to check the protector once in a while to make sure it is still OK.
They have a finite life, in terms of number of hits, intensity etc. Probably
not a bad idea to replace it once in a while, if you value your premium
gear.

NOW, if you should be unlucky enough to suffer a direct hit, its a whole new
ball game. A lightning stroke typically provides a current of 20KA, with 10%
of strokes providing 200KA or more. A little calculation with ohms law will
tell you that even if you achieve a station earth as low as 1 ohm, you will
still get a potential rise of typically 20KV. You can do the sums for
different values of earth resistance and lightning current. This is why you
should bond every metal appliance and fitting etc to your station earth
system. Otherwise, there will be huge potential differences at the time of
the strike with possible disastrous consequences. This is also why your
telephone line represents a real problem at the time of a strike - it
introduces a remote earth at nominally zero potential into your house, at
the instant when the rest of the house is at many KV. Moral - stay off the
phone during storms, or use a cordless.

By the way, achieving a 1 ohm earth is a VERY difficult thing to do, even if
you live on top of a salt water swamp. A 1 ohm earth is normally only found
in power switchyards and the like where they have extensive earth mats.

Also, current practice on lightning protection is that a tower or tall
building does NOT provide a cone of protection, but only a long tube. The
current standard on lightning protection here in Oz has been amended to
include this. I used to live near a TV tower and we regularly saw it get
hit, including on the side. If it had a cone of protection, that could never
happen.

Lastly, once lightning is travelling along a downconductor, it does not like
to go around corners. This is because corners represent an inductance, and
with so much current, the voltage build-up even in a right angle bend in a
conductor can be quite high. The strike waveform contains significant power
at frequencies running well into the megahertz, so lightning downconductors
should be as straight as possible and provide a direct path to the earth
mat.

Regards,
George Georgevits
VK2KGG