A Steel over Copper conductor -

At very high frequencies current will flow only on the outer surface
of the steel according to the conductivity of steel.

As frequency decreases, current will flow in the copper only when skin
depth in the steel is greater than the thickness of the steel
covering.

As frequency decreases further current will begin to flow deeper in
the copper according to the conductivity of copper.

The resulting resistance of the composite structure is the resistance
of the steel layer in parallel with the resistance of the layer of
copper, taking the currents flowing in each layer into account.

But at HF, unless the steel layer is microscopically thin, the
resulting resistance will be practically the same as that of a solid
steel conductor.
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Reg, G4FGQ