Take a dummy load and attach a piece of wire on some kind of a windup
system. To increase the efficiency, let out more wire and vice versa.

Good Idea, and we want to decrease efficiency, so the wind-up system may just
may be what is needed. What I was looking for is antenna materials that
decrease efficiency.
If we could come up with something as bad as the B&W dipole for a lot less
money, then we may have something.
73 Gary N4AST