BobK207 wrote:
If a small diameter ground rod (1/2" or 5/8") doesn't give the 25 ohms
(or less) to ground.....would a larger (3/4" or 1") diameter rod in the
same soil do better?

Is the soil the culprit or the soil / rod contact resistance? The
larger diameter rod contacts more soil..better grounding behavior?

cheers
Bob

Having worked electronic facilities where the ground resistance has to
be less than 5 ohms, I've tried a lot of tricks to get the resistance
down. There is no sure-fire trick to the process, but rod diameter is
WAY down the list of possibilities. The number of rods (up to 12) and
the depth of the rods (sometimes 40ft) is the usual key, but if that
doesn't work try pouring 25lb of salt in a hole next to the rods and
water it good. Salt is a good conductor & decreases ground resistance.

Bob S.