I sent too soon.

The problem with ground rods at high radio frequencies is that depth of
earth renetration may be low as compared with length of the ground rod.
Contact that counts is that which connects with the current path.

There is a special problem with the aluminum material itself of the
ground rods mentioned by the questioner. Aluminum, zinc, and magnesium
are galvanic anodes used for cathodic protection of less active metals.

A copper ground rod is likely to last forever in the soil. An aluminum
ground rod is likely to soon be sacrificed in its attempt to protect
other connected metals in the same galvanic soup.

Best regards, Richard Harrison, KB5WZI