[time-nuts] GPS antenna and lightning

Brooke Clarke brooke at pacific.net
Sun Oct 4 13:00:24 UTC 2009


Hi Kevin:

I agree with all you have said regarding how the wiring should be to minimize 
currents in equipment, but . . .

It is possible to greatly lower the possibility of lightening striking some 
location.  It's done by using what amounts to bottle brushes made of metal that 
are about 3" in diameter and a few feet long.  You place these below and around 
the GPS antenna and make a good connection to ground.  They bleed ions from the 
earth into the air forming clouds (pine trees do a similar thing).  A number of 
the brushes are used to protect an area. If you want to attract lightening 
replace the bottle brush with a metal sphere.

Note old fashioned "Lightening Rods" do not work as well as the brushes.

For example see:
http://www.lpsnet.com/ALS.asp
http://www.allteccorp.com/prod_terrastat.php

Have Fun,

Brooke Clarke
http://www.prc68.com

kevin-usenet at horizon.com wrote:
> Although not exactly trivial, it is actually not only possible but
> surprisingly easy to design a setup that can indeed withstand repeated
> direct lightning strikes without damage.
> 
> Large AM transmitters deal with this all the time.  Polyphaser
> sells a lightning strike *counter* so you can schedule maintenance
> on your lightning protection:
> http://www.comm-omni.com/polyweb/lsc.htm
> 
> What you have to remember is to use electrical Aikido.  There is
> nothing you can do that will possibly STOP a lightning strike, so don't
> even try.  Your goal is to DIVERT the strike away from passing through
> your equipment.
> 
> Some people really seem to have a hard time abandoning the mental image
> of "stopping" lightning, so please forgive me as I hammer the point home
> with several different examples...
> 
> 
> It's not unlike dealing with explosives.  A *typical* lightning strike
> dumps 500 MJ of energy, or 0.12 tons of TNT equivalent.  Wrapping that in
> steel is called a "fragmentation grenade" and just creates more damage.
> Explosives storage facilities don't try to contain that; rather, they
> have strong walls facing directions to be protected and weak walls (and
> roofs) in other directions so the blast can escape before damaging the
> strong walls.
> 
> Consider a lightning strike to be the closest thing to an ideal current
> source you are going to encounter.  You *cannot* stop it with series
> impedance alone, no matter how high; you have to provide it with a very low
> shunt impedance.
> 
> Likewise, remember that it's not the volts that kills you, it's
> the amps.  It doesn't matter if your entire equipment bench bounces
> 1 MV, as long as all potential *differences* are small emough that
> no damaging currents flow.
> 
> The basic layout is the "moat and drawbridge" illustrated at
> http://www.sigcon.com/Pubs/news/7_02.htm
> 
> Lightning will take the lowest-inductance path(s) to ground.  Your goal
> is to make sure that all paths through your equipment pass over a SINGLE
> drawbridge, which is tied together with surge-diverting devices such as
> spark gaps, gas discharge tubes, MOVs, transzorbs, etc.
> 
> Separate the stages with a bit of series impedance like unsaturatable
> air-core coils.
> 
> In a typical AM transmitter shack, the drawbridge will take the form of
> a big well-grounded steel plate on one wall sized to handle kilo-amp
> currents without damage.  All wires, WITHOUT EXCEPTION, entering or
> leaving the shack pass through feedthroughs in that steel plate.
> 
> Any damaging current would have to pass over the drawbridge, loop through
> your equipment, and back to the exact same drawbridge to ground.
> This is a dead-end path that current is not going to flow through.
> 
> It's not hard to make the high-frequency impedance of that loop something
> like 1M times larger than the direct path through the steel grounding
> plate.
> 
> Then, of a large but not extraordinary bolt of 100 kA, only 100 mA goes
> through your equipment.  Suddenly, it's a lot less threatening.
> 
> 
> But the secret is making sure that *every wire* to your protected
> equipment has a low-impedance path to *every other* wire.  It's not a
> matter of protecting them individually, because the protection does not
> STOP lighting current.  You have to tie them all to the SAME ground point.
> 
> For example, if you have equipment plugged into two different surge
> suppressors, you can have lightning pass in one and decide to send half
> of its energy out through the other via your equiment.  On the way,
> it lets out all the magic smoke.  :-(
> 
> You have to consider the impedance between each possible pair of wires.
> Where is the shunt path, and why is its impedance many orders of magnitude
> lower than the path through the protected equipment?
> 
> Just one little wire that's not tied into the system provides a path
> that will let damaging currents come in through any other wire, no matter
> how well "protected" they are.
> 
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