The more expensive Sanwa RXs like the one that comes with the MT-4 use co-axial antenna wires and you have to be very careful with this as it's easily damaged.
It may be that the person you bought it from didn't read the manual and put a sharp bend in the antenna wire (which is the very thing that destroys them) and then decided to sell it on but his loss could be your gain if your soldering skills are up to fitting a new one
