How to place a coaxial cable in my simulation model to excite a SICL antenna, help!

I am trying to simulate the SICL structure design with reference to the paper (attached PDF file) in Ansys HFSS. In my simulation design (attached image) I would like to place the Coaxial feed in a substrate integrated coaxial line(SICL) feeding technique. But for using the Coaxial feed, the confusion is which one conductor is to be considered as a ground plane for such structure. I am new one to SICL. Kindly help me.