Part of the problem that everybody gets into when
discussing lightning and our radio gear is the notion
that lightning is somehow "seeking..." or "comes in
on" one thing or another. Perhaps a better way to
visualize it is to think of the whole lightning
problem as being one of energy transfer. So the name
of the game becomes making sure that your lightning
protection plan provides a means to transfer all of
that energy safely around your gear and/or make sure
that little or none of that energy flows *through* any
of your gear.
A nearby by strike will raise the local voltage
gradient by hundreds if not thousands of volts. This
includes all objects near the strike zone, electronic
or otherwise. But as long as *all* of your gear and
other surrounding objects are all rising and falling
together there will not be any damage. A lot of what
people do for "grounding" is actually achieving just
that, keeping thier gear in sync with its surroundings
as the voltage rises and falls through the strike
event.
This is also what happens in the case of aircraft when
they get struck by lightning, they don't have any
"grounds" after all do they? So when they are hit by
lightning the entire aircraft voltage level rises to
essentially equal the lightning's voltage potential,
and then it falls again after the strike event.
Lightning will generally ?attach? at one point near
the front of the aircraft and will then "sweep" across
the fuselage with many more successive attach points.
There is also a corresponding point on a trailing
surface or edge (typically) that completes the
lightning current path (it generally doesn't sweep).
It is also not uncommon for the lightning to "sweep"
completely across the airframe and then rejoin with
itself after the airplane has passed completely
through the strike channel.
So the goal in aircraft lightning protection design is
to make sure that the lightning energy is conducted
through the airframe as efficiently as possible while
preventing significant amounts of it from coupling
into its internal systems like electrical, fuel,
controls etc. It is when the lightning energy directly
flows or is electromagnetically coupled into those
various systems that an aircraft gets into big
trouble. Or if there are significant voltage potential
differences between the various components of those
systems.
So why ponder aircraft when dealing with our shacks?
And how does this relate to our radios? Primarily by
the fact that "ground" (earth) really isn't, it is
resistive at best. And that the "local" ground (earth)
will rise in voltage potential, not unlike the
airframe of an aircraft during a strike event. So the
protection goal for the shack is to safely conduct the
direct currents around the gear, keep voltage
potential differences to a minimum, and to also
minimize the amount of electromagnetic coupling that
there is from the "indirect" effects of lightning. And
as it turns out the same practices that eliminate
various RFI issues also help address lightning?s
electromagnetic coupling issues (indirect effects). So
chances are if you have problem with RF getting into
things when you operate you will also be more likely
to suffer lightning damage even if it is not a close
strike.
As noted below the electric power (and telephone line)
side of the equation is often overlooked or under
addressed when designing a station's lightning
protection approach. It is also true that the
electrical and power lines represent "grounds" that
extend far beyond the immediate vicinity of our
shacks. That is why they appear to be a source of
lightning "getting into" our gear. They are in reality
just a relatively low resistance path for the energy
to try and use to equalize. Where the problems come in
is that they don't handle the high peak currents and
inductive reactances of a lightning event very well.
And if you gear is in between your antennas (or local
ground system) and the power or phone system (wiring)
it becomes part of that path with often disastrous
results.
Hopefully this is a slightly different way to think
about and mentally visualize the lightning protection
problem. And if you really think about all that I've
presented here it is actually very consistent with
Pollyphaser and the various other vendors who deal
with lightning protection.
Duane
N9DG
--- Ken Brown <ken.d.brown@verizon.net> wrote:
> My experience (which includes over 20 years of
> working on radio sites or
> observatories on mountain tops) indicates that the
> above is absolutely
> true. It is true even when the radio site is the
> highest mountain for
> miles, and has antenna towers holding antennas 60 or
> more feet above the
> summit, AND less than proper grounding and lightning
> protection on the
> antenna feedlines.
>
> Why?....It is simply a matter of probability. The
> power grid covers a
> much larger area, and therefore has a much higher
> probability of getting
> a direct hit.
>
> Does this mean we should not bother with lightning
> protection on our
> antenna systems, and just put the protection on the
> incoming power
> lines? Absolutely not! Just don't forget the
> protection on the power
> lines, it is very important.
>
> DE N6KB
>
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