Surface conductivity of anodize aluminum

We're using anodize aluminum for antenna.
To feed it, I use a piece of copper. The surface of copper lay on surface of aluminum element. I doubt the conductivity of anodize will give some loss.

I could clean the surface by some kind of acid like HCl. Does it help?

Pls advise.

The oxide layer is insulating. There are different ways to make a long-term reliable connection to anodized aluminium. Grinding it off would be a simple method, probably better use a tooth washer between the parts, it penetrates the oxide layer when the connection is fastened.

A standard anodized surface on aluminum is insulating but if you use lock washers or grind the anodized surface before attaching the copper part, the oxide layer on aluminum is broken and causes no problems. I estimate up to 10 GHz there are no problems with anodized aluminum.
In microwave design, iridiated aluminum is used as corrosion protection and is conductive well up to 100 GHz.

the anodize layer is so thin, that is will not impede the operation of the antenna as a radiator. The rf will go right thru it. Like stated above, your only trouble is to make the initial electrical contact thru the anodized layer. If it were me, I would selectively plate nickel over the small patch where you are attaching the conductors to the aluminum. If just just using a star washer to cut thru, I would advise using a corrosion preventative gel, like they use on AC house wiring, to keep the aluminum from oxidizing again.