In many areas, it is not a good idea to use aluminum wire to expand your ground system due to corrosion loss of the aluminum metal in direct contact with the soil. Aluminum is very electronegative compared to most other metals and will become sacrificial as a buried component in your ground system. In fact, aluminum is sometimes intentionally used as a buried sacrificial anode for cathodic corrosion protections systems. (Thanks, Matt KM5VI)
When I was new to Part 15, I didn't know about
copper poles for transmitting antennas. The first
transmitting antenna was an aluminum TV mast.
It's amazing what I didn't know. And it weighed a
ton. The ground system I put down years ago
involved all different kinds of wire. Some wire
had insulation, and some didn't. It worked well,
though. (I bet it wouldn't now.)
Good to know
Bruce
A cable TV system not far from here found out the hard way about buried aluminum. Much of there system cable was direct buried aluminum hard line. After a few years they were deluged with complaints of poor reception. Yep, all that buried cable had mostly disintegrated.
Now they use jacketed cable.