Rick (W-A-one-R-K-T) wrote:
In the 2007 ARRL Handbook page 3.6, there is a discussion of building a
ground ring around the base of the tower, connected to three ground rods
each connected to each leg of the tower.

In the narrative it says "Space rods at least 6 feet apart" but then in
the caption for the accompanying drawing it says Locate ground rods on the
ring as close as possible to their respective tower legs".

On a Rohn 25G, the rods can't be "as close as possible" to their
respective tower legs and still be anywhere near 6 feet apart.

So, which is it ... close to their respective tower legs, or 6 feet apart?
probably the 6 feet apart, but it doesn't make a heck of a lot of
difference..
one approach gives you a lower ground resistance
the other approach gives you shorter wires from tower to ground rod,
which has less inductance.

And, if your tower is going to be sitting on a big block of concrete and
rebar, and you put a suitable copper wire in the concrete, that is
probably a better ground than any set of rods, because the surface area
is huge. (unlike a rod, where you've got all of 1/2 a square foot or so
surface area)


What is the reason for the 6-foot separation, anyway? The rods all go
into the ground. Why would it make a difference if they're closer
together than 6 feet?


Consider if you drove two rods two inches apart.. it would have the
ground resistance of one rod (or very close to it). If you separate the
rods by their length (or thereabouts), the effect is to almost halve the
ground resistance. Separataing by much more than a rod length doesn't
change things much.

Think of a rod as not just a rod, but a sort of squashed hemisphere.


if you want equations for all sorts of configurations:
http://home.earthlink.net/~jimlux/hv/grounds.htm

mostly taken from IEEE Std 142