The Collaborative International Dictionary of English v.0.48:

sign off \sign` off"\ (s[imac]n` [add]f"), v. i.
   To terminate a communication, especially one conducted by
   radio waves; to terminate a broadcast over radio or
   television; as, this is your reporter in Cairo, signing off.
   [PJC]

WordNet (r) 3.0 (2006):

sign off
    v 1: cease broadcasting; get off the air; as of radio stations