two diametrically opposite sections on said sleeve, each of said sections having recess means on said inside surface opening toward said axis for receiving one of the transmitter and the receiver and to permit {TAIL ~ radiation} to pass through the body between the receiver or {HEAD ~ transmitter}; and 
two diametrically opposite sections on said sleeve , each of said sections having recess means on said inside surface opening toward said axis for receiving one of the transmitter and the receiver and to permit { TAIL ~ radiation } to pass through the body between the receiver or { HEAD ~ transmitter } ; and
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH