Z0 of a line with unequal wires

In case of a transmission line consisting of two wires in parallel (aka Balanced line) the characteristic impedance Z0 is given by this equation:



where l is half the distance between the wire centres, R is the wire radius and \mu, \epsilon are respectively the permeability and permittivity of the surrounding medium.

I have two questions I cannot answer myself:

• what is the characteristic impedance in case of wires of different diameter ?

• In case of a tube rather than a solid wire: does the characteristic impedance depend soley on the outer diameter or does the inner diameter play a role as well?

Thanks in advance for any hint.
Ulli