[time-nuts] lightening protection of a GPSDO system / optical isolated distribution amp

Jim Lux jimlux at earthlink.net
Wed Nov 26 21:21:20 EST 2014

On 11/26/14, 5:23 PM, Chris Albertson wrote:
> On Wed, Nov 26, 2014 at 4:03 PM, Jim Lux <jimlux at 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 

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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