RF Cavity Coupler Acting Like Twinax

Hi,

I am currently working on a loop coupler, to couple out magnetic HOMs, and due to the loop I am thinking of modelling the returning loop section as twinax joined at the ends. Can anyone say wether this would be sensible thing to do, or should I look at a modelling it (with equivalent circuit) in a different way.

If Twinax seems appropriate I was wondering if anyone could tell me if there would be a capacitance between the two coaxial lines (Twinax) if they are joined at the ends (i.e. loop coupler)?

Thanks in advance.