Dear h_cet,
For your situation, maybe what you can try is:
At the input end to your wire antenna, draw a coax and assign it as a waveport that you seem to know of.
At the termination end of your wire antenna, do a small break just before reaching the end of the wire. Then at the break, assign a lumpedRLC 50 ohm boundary.
I really don't know whether this will work, because the fact that you don't even have a finite sized ground plane still puzzles me.
Hope this helps,
Dave