On 11/26/14, 5:23 PM, Chris Albertson wrote:
On Wed, Nov 26, 2014 at 4:03 PM, Jim Lux <jim...@earthlink.net> wrote:
On 11/26/14, 2:14 PM, Chris Albertson wrote:
You CAN (almost) lightening proof your system. The trick is to give
lightening a low impedence path to grind at very opportunity.
Start with the antenna mast and call. Use iron pipe for the mast and feed
the antenna cable down the center of the pipe. Place two large ground
clamps on this pipe and connect a large diameter wire that takes a
straight
path to a group rod. This will go a long way to diverting energy to
ground because high voltage likes to flow on the outside of a conductor
which would be the pipe and not so much the antenna cable.
Not so much high voltage, as AC and skin effect. However, bear in mind
that lightning has a rise time of a microsecond or so: you can think of it
having a fundamental of 300-500 kHz (e.g. the first quarter cycle of a
sinewave), with most of the power below 1MHz.
Skin depth at 1 MHz in copper is 0.065mm.
In iron (using conductivty of 9.6 and relative mu of 1000) skin depth is
0.005 mm
So, steel/iron pipe is a terrible conductor for a lightning impulse,
compared to that nice copper coax next to it, or inside it.
Really? What you care about is the impedance, not the depth of the skin
effect. Also not the the coax is NOT exposed to the environment. Tacitly
the coax comes out from the user side of the antenna, so it never sees
daylight. The current flows in that first .005mm of steel. Running the
coax down the side of the pipe would be a really bad idea.
Skin effect greatly affects the resistive impedance.
Compare a copper pipe vs a steel pipe of the same diameter.
The copper is basically conductive ring 0.065 mm thick and resistivity
of 1.67 and the steel is .005mm thick and resistivity 9.6
So the resistance ratio is 26:1920.. that is 2 orders of magnitude.
However.. the dominant impedance at lightning frequencies (at least for
copper) is the inductance, which is very weakly dependent on the shape
and cross-sectional size of the conductor: it's close to 1 uH/meter
regardless..
A copper pipe that is 2 cm in diameter and 1 meter long has a resistance
at 1 MHz of 0.004 Ohm
A steel/iron pipe that is 2 cm in diameter and 1 meter long has a
resistance at 1 MHz of 1.43 Ohm
The inductive impedance is about 6.3 ohms.
So the steel has a somewhat higher impedance than the coax inside it.
Yeah, there probably is some shielding effect, but I'm going to guess
that there's some insulating gap between the "bottom of antenna" and
"top of pipe", although that gap may be bridged by the plasma from your
lightning strike.
It's just that I'm not sure I'd trust the "shielding effect" of the
pipe, nor any preferential current distribution from the magnetic
fields. For all one knows, that pipe might wind up filled with a plasma
of vaporized coax.
I'd just consider the antenna and feedline sacrificial, and worry about
dealing with the transient at the point of entry to the building,
assuming that the coax is carrying all of it.
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