You can use the Vector Fitting (VF) algorithm to obtain the equivalent circuit of a complex structures like antennas or packages. However, there might be some issues like obtaining the non passive or non causal equivalent model. Use the S_parameter of your antenna as the input for VF algorithm. The output would be the poles and residues of the equivalent circuit and then you can find the proper RLRC values for the equivalent circuit.
From HFSS for the structure as shown and similar you can get scattering matrix which can be used in some circuit simulators in frequency domain. There is method to convert scattering matrix into equivalent circuit which can be then simulated in either frequency or time domain.