"I have a dipole with 134 feet of 20 gauge insulated copper wire. What
would happen if I wanted to replace one leg (67 feet) with 18 gauge
copper clad insulated steel wire but chose to keep the other leg with
the 20 gauge wire?"

It would be interesting to know what is the reason: corrosion or an
something else.

Also if the leg is the antenna or the counterpoise.

I agree with Bilou: "Joel now has a quarter wave over a ground plane.A well
known case.
Impedance is halved .
Resonance is only dependant of the remaining wire and unchanged."

What materials are recommended for radials (counterpoise) and for the
radiated arm?
S*