When you think about it, a single rod stuck in the ground doesn't provide a
very good connection to "ground". The surface area of a 1/2" diameter rod 10
feet long is only about 190 square inches. A 10x10 inch plate will have 200
square inches. Granted, due to finite conductivity of the ground, the
impedance to a presumed "far field" doesn't scale quite that nicely, but you
get the picture.
In most concrete slab/foundation construction, they use a "UFER" ground, which
is basically a wire embedded in the somewhat conductive concrete slab. The
slab, of course, has a very large contact area with the surrounding ground.
In practice, too, the DC resistance isn't as important as the RF resistance (in
the case of a vertical antenna), or, in the case of lightning protection, the
inductance.
Jim
W6RMK
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