How to simulate the antenna?

The question looks relatively simple. I would start by building a model of the coax, probably exciting it with a modal waveport at one end for sure and maybe a second port at the other.

The one twist to consider is that the coupling (radiation) may be a small part of the input and that will probably necessitate a tightening of the convergence criteria (e. g. the problem size and RAM requirement will grow in entertaining ways).