Eric/Nick:
Both of you are right about not trusting the "cone of protection." It is
widely used in the power industry for the design of shield wires on HV/EHV
transmission lines and in substations. Where that concept came from was a
set of tabletop experiments conducted by Westinghouse several generations
ago. However, it is well known within the power industry that the
"shielding angles" are only valid for geometries very close to those in the
Westinghouse test setup.
Then it gets worse. The shielding angle only gives you a prediction of
probability of a hit. It is more probable outside the cone of protection
than inside. However, one actual hit no matter how improbable will ruin
your whole day. BTW, you cannot really design to survive a direct hit. If
your antenna is actually struck by lightning, the antenna itself will most
likely be destroyed, along with the coax and the rig. Also the building
will suffer structural damage.
Inductance comes in two flavors and both of them will bite you. The pulse
of current in a lightning stroke is a broadband signal with a peak in the
neighborhood of 500 kHz. The problem is that the reactance to a 500 KHz
signal arising from the self-inductance in a long ground lead may be high
enough that the surge might seek a lower impedance path to ground, like
maybe through your rig. That is why you need to have a short direct
connection from your transmission line to the ground rod located well away
from the rig.
The other problem is the one that Nick refers to, mutual inductive
coupling. A wavelength at 500 kHz is 600 meters (and a sixth of a
wavelength is obviously 100 meters) This is important because, for
electromagnetic effects, the induction field is significant out to 1/6 of a
wavelength, and trails off rapidly further out. In other words, if
lightning strikes anywhere within 100 meters of your station, a replica of
the wave will be coupled into every conductor in your station. In the worst
case scenario (a super stroke) the peak current can be several hundred kA.
The coupling is inefficient; thus, maybe only a few to a few hundred amps
gets coupled into your station. This is what happens when your rig gets
burned up, but the foundation of your house did not crack. Near misses can
be protected against (somewhat) by short direct ground connections.
73
Steve Kercel
AA4AK
At 09:24 PM 9/1/2005 +0100, Nick Waterman wrote:
Eric J wrote:
There is a "cone of protection", they say, around a high point with an
angle of 45 degrees. I wouldn't tempt it myself. However, I'm near the
base of a 1900' peak and I've watched lightning hit the peak, but have
never seen lightning anywhere near the area surrounding the peak. I
believe in the theory, but still...
There's still this thing called INDUCTANCE, and I've not done the
maths, but 25 kiloAmps (there's a unit you don't often use!) in one
"wire", maybe 100m away from the wire connected to your rig...
The web has figures around 500 GW. Would you let someone transmit that
kinda power into a 3 mile antenna within a mile of your expensive kit? No
thanks!
--
"Nosey" Nick Waterman, Senior Sysadmin.
#include <stddisclaimer> [EMAIL PROTECTED]
False hope is better than no hope at all.
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