This is the Sanwa RX-481 2.4Ghz FHSS-4 4-Channel Surface Receiver, with a built-in Internal Antenna. The RX-481 Super Response receiver features a built-in antenna! This means no more antennas sticking out of the body, as well as no more worry about damaging the antenna during use. Just mount the RX-481 Super Response receiver in the upright position in your model and enjoy the same solid reception as you've come to expect from other Sanwa surface receivers. 

Sanwa 4-channel RX-481 Receiver w/ built-in Antenna