Hi Howard,

The purpose of the ground rod it to get down to where the soil is wet, where the ground is conductive.  Around the Pacific Northwest that is typically about four feet.  That said, you still should try to get the rod as deep as you can. Otherwise you may see performance differences depending on the season as the water table goes up and down.

Oh and a good tool for pounding the rod into the ground is a T-Post Driver.  It is a 2 1/2" capped heavy pipe with handles welded on each side.  Much easier to pound the rod into the ground than using a hammer.

Jim
W7ABD