Putting in some new ground rods today.

Dirt in the yard (Maryland) is semi-rocky. House is on a hill and
certainly well above the water table.

I sank 5/8" copper-clad 8-foot ground rods around the yard. Some went
in fairly easy others required some pretty good sledgehammer action.
No two ground rods are closer than 16 feet apart.

Random measurements between two nearish (20 to 25 feet) ground rods
give resistances in the 50 to 100 ohm range. The ones that went in
easiest seem to have highest resistance. (Possibly construction fill
near the house foundation?)

Paralleling a bunch on one side of the yard, and a bunch on the other
side of the yard, gives resistances between the bunches in the 15-20
ohm range.

Resistance measurements done with homebrew fall-of-potential tester
that puts about 100mA between the probes. Tested both polarities and
results seem similar. If I scope the potential between the probes
there's clearly already a fair amount of 60Hz AC current flowing in
the ground as well as a DC component (there is a substation across the
street and about 100 yards down if that means anything...)

I'm guessing this means that when I tie everything together I can
guesstimate the resistance to "true" ground being in the 10 to 20 ohm
range.

I kinda wonder why I spent money on all this heavy 6 AWG copper wire
to hook everything together, when the resistance of the ground itself
is bigger than the resistance of the wire. I mean, it wouldn't feel
right wiring it with 22 gauge hookup wire, but isn't 6 AWG overkill?
10 Ohms of 6AWG is like 5 miles of copper that I can't afford :-(.

Tim KA0BTD