[time-nuts] GPS antenna selection - lightning

Bill Hawkins bill.iaxs at pobox.com
Sun Aug 7 23:06:56 EDT 2016


This thread grows old, so here's one person's summary:

There are two ways to be damaged by lightning:

1. A direct hit pumps 100 kiloamps of electrons into an ohm or so of
your local wiring. There is no way to survive a direct hit except to
implement stuff only the Military can afford. The probability is so low
(outside of Florida and mountain tops) that your homeowners insurance
may cover it.

2. A 100 KA strike goes to ground near you, with two effects:
  a. The ground resistance allows a large range of volts per meter to
kill cows but not golfers with their feet together.
  b. A mighty electromagnetic pulse (EMP) induces voltages in anything
inductive that is not shielded or twisted.

Case 'a' argues for a single point earth ground. When the ground voltage
goes up, you want all of your equipment to go up with it, as if it was
on an isolated ground plane. It seems best to use the Electric Power
Company's house ground for that reference point in your home. If you use
a UPS for a set of equipment, everything on it should ground to that UPS
(which should have a high capacity surge arrestor). You are left with
telephone cords, TV cables, and antennas as peripheral connections to
protect with surge arrestors. Marine supply stores sell rolls of 4 inch
wide copper strap for connecting the mast on the wheelhouse cabin with
the keel of fiberglass boats. This is also the ground for all electronic
equipment. The strap is considerably less inductive than a wire.

Case 'b' argues against long wires inside the area that contains the
common ground and the surge arrestors at its periphery.
Surge arrestors have energy ratings that refer to the energy of the EMP
that caused the surge. I have no idea how that relates to lightning EMP
energy so I buy the most capacity I can afford.

I used these principles in a home that had a pair of HP GPS antennas
four feet apart on a twenty foot mast of six inch plastic pipe, using N
connectors and 50 feet of RG-8 to a pair of Z3801A receivers. The
neighbor's tree took a direct hit (was split apart) less than 100 feet
away. He had extensive electrical damage originating at the outdoor
flood light six feet from the tree. I lost the GPS antenna closest to
the tree but nothing else. FWIW, I had wireless G access points
separating the area connected to the antenna from the rest of the house
network. No attempt was made to beef up the grounds.

Regards,
Bill Hawkins




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