christofire wrote:

Isn't aluminium in the presence of air naturally coated with a thin layer of
aluminium oxide which doesn't conduct? In which case aluminium might be
fine for lightning conductors but hopeless for RF. What will the tower
carry? ... if it's all VHF and upwards then there's probably no problem.

Chris

Yes, aluminum is immediately coated by a few-molecule-thick layer of
aluminum oxide when exposed to air. It's a non-porous, brittle, ceramic
material commonly used for hybrid circuit substrates and for sandpaper
grit, among other things. It's why aluminum, despite its extreme
chemical activity, doesn't corrode -- unless the environment is capable
of dissolving the aluminum oxide, which some are. Aluminum oxide is an
excellent dielectric, into at least the microwave range.

But an insulating film doesn't make a conductor "hopeless" -- after all,
the most perfect bare conductor is surrounded -- "coated" if you will --
by air.

Nor does a highly conductive coating degrade a conductor's performance.
Only a layer of poorly conductive material of sufficient thickness is
detrimental.

Roy Lewallen