As Alienhead says..
I think a simple antenna would be use null headphone jack from Radio shack, with pieces of wires soldered to Left and Right pinouts. or use some old headphone cable and let a few cms of wire stick out.

On the RX-51 Full schematics, page 8 ..
the FM antenna sources signal from the L and R pins of the headphone connector and sends to BTFM Module.