[time-nuts] GPS antenna and lightning

Don Latham djl at montana.com
Sun Oct 4 17:19:35 UTC 2009


Brooke:
NO NO NO. these devices are hokum. It's true that sharp points will
release shielding charge under a strong electric field such as under or
near a thunderstorm. But think what happens to that charge under any kind
of wind-it goes away. Also, the shielding effect of the charge under
no-wind is simply a small sphere. Also, if you have to try such a thing,
just get a little bunch of barbed wire...don't pay some joker an arm and a
leg for it.

Kevin is substantially correct. first, and most important, lead the
lightning where you want it. Second, it is impedance, not just resistance,
that is important. The risetime of the remote pulse is about 1
microsecond. But, the direct "hit" can generate pulses with much higher
risetimes. Also, having a single ground point is the point :-) (I'd put
mine outside the shack on the wall, BTW) - just consider the current that
flows through an 0.01 ohm impedance with 30 kv driving it. Don't depend on
the power system ground for ANYTHING to do with lightning with respect to
your ham gear.
Third, many of my friends have done serious, peer reviewed work on
lightning rods. Turns out that a 1/2 or 3/4 inch rod with a hemispherical
end AND A LOW IMPEDANCE GROUND PATH is the right thing for managing
strikes; radioactive or brush "rods" only serve to protect themselves and
don't do a darn thing for anything around them except divert the strikes
to the close surroundings.
Don Latham

Brooke Clarke
> 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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-- 
Dr. Don Latham AJ7LL
Six Mile Systems LLP
17850 Six Mile Road
POB 134
Huson, MT, 59846
VOX 406-626-4304
www.lightningforensics.com
www.sixmilesystems.com




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