Lets start by first adjusting the perspective. CG lightning
seeks charges located 5 kilometers over there. Therefore the
shortest electrical path is 3 kilometers directly down to
tree, antenna, or utility wire. Then 4 kilometers through
earth to those charges. Where will lightning strike earth?
Answer is better found in geology. What is the more
conductive earth? Has a transcontinental pipeline changed the
electrical nature of that region? Obviously the answer is not
always obvious. Then many assume lightning is capricious.

Some campers slept near a tree when that tree was struck.
Those sleeping tangential to the tree were not affected.
Those sleeping pointed at the tree required medical
treatment. An electrical path was down tree, through earth,
up into human body at the feet, out of human body at the head,
then onwards to earth borne charges located elsewhere.

IOW, the camper who required medical treatment were, in
essence, earthed at two separate points (at foot and head).

Suppose we had surrounded the camper with a buried electric
wire loop. IOW the conductive path was around the sleeping
camper. A single point ground (equipotential) was created
beneath the camper.

Single point ground can be a single earth ground rod at one
of the building, or it can be accomplished by making earth
beneath equipotential. But it comes back to a basic concept.
No current flows if an incoming and outgoing path does not
exist. Goes right back to elementary school science that
defined electricity. First a complete circuit must exist
meaning that each component must have both an incoming and
outgoing path.

The concept of single point ground is make only one
connection so that both incoming and outgoing paths do not
exist. Some examples of how this is accomplished:
http://www.erico.com/public/library/...es/tncr002.pdf
http://www.cinergy.com/surge/ttip08.htm

http://lists.contesting.com/archives.../msg00644.html
http://lists.contesting.com/_towerta...il/004413.html
http://www.leminstruments.com/pdf/LEGP.pdf (see page 14)

How multiple earth grounds can cause damage to a fax:
http://www.epri-peac.com/tutorials/sol01tut.html

Notice even a buried wire can carry destructive transients
as demonstrated by the phone wire in that erico.com figure.
Even a buried wire must connect to the single point earth
before entering a building so that all wires enter at the
single point:
http://www.polyphaser.com/ppc_TD1023.aspx

wrote:
I am trying to gain a better understanding of the issues associated
with lightning and grounding, particularly for an amateur radio
station. I would greatly appreciate any answers to my questions, given
below.

1.) It seems like lightning "finds" a path before actually discharging
through it. Is this correct? How fast is the process of finding the
path?

2.) Does lightning discharge over one path or many paths? In other
words, if lightning strikes a point, and there are two paths from that
point to ground, is the energy shared - like a current divider? One
would think that this is what happens, with the sharing based upon
relative impedances to ground.

3.) This talk about "single point ground" can be confusing, especially
when the solution involves multiple ground rods. I believe we are
really saying that the station (equipment) should be connected to
ground at only one point - but you can have any number of ground
connections (i.e., like ground rods) otherwise.

4.) Does the size of an antenna affect the probability of its being
struck by lightning, all else being equal? What is the governing
relationship?

5.) Finally, consider an amateur antenna on a pole, located some
distance from the service entrance of the house. Let's say there is a
copper cold water pipe right at the service entrance. Is it wise to
place a lightning rod (or rods) right under the antenna, place an
arrestor on the coax there and then another arrestor where the coax
enters at the service entrance? Is a ground wire needed between the
remote ground rod and the cold water pipe?

Thanks in advance,

-JJ