[time-nuts] GPS antenna and lightning

Rex rexa at sonic.net
Sun Oct 4 10:43:16 UTC 2009


That is a very informative description.

I've not lived in a lightning-probable area with antennas. Here 
lightining is a remote possibility, but I'm trying to digest what you 
have told us.

You said, "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."

I don't get exactly what this means.

Say I have two or three radios, each with a coax running to an antenna. 
Obviously I would run them all through the well grounded plate you 
describe. My thought is that each of them should have a lightning 
supressor feed through right at the plate, not sure if I have the 
terminology right, but a comercial arc-over (probably gas filled) 
lightning supression device.

So the different supressor problem you mention is what confuses me. Does 
the configuration I mentioned for multiple radios, have something to do 
with what you say is a problem, or is it something else. If what I 
described is a problem, how can one handle multiple feeds?


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:
>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
>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
>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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