go off the air
Synonym discussion
Sense: phrasal verb/1
All three describe a broadcast ending, but with different nuances. 'Go off the air' is the neutral, standard term for any broadcast that stops — scheduled or unexpected, temporary or permanent. 'Go dark' is more informal and dramatic; it often implies an abrupt, unexplained, or permanent shutdown of an entire station or network, rather than a daily schedule ending. 'Sign off' focuses on the human or ritual aspect — the host says goodnight and the station closes its transmission for the day. For example: 'Henrik's show signs off with his catchphrase every night'; 'When the local newspaper's radio arm went dark, no one had expected it'; 'The channel went off the air at midnight after forty years of service.'
Etymology
This phrase comes from early radio broadcasting in the 1920s, where 'the air' referred to the invisible medium — radio waves — that carried sound from the transmitter to listeners' radios. A station was 'on the air' while transmitting and 'went off the air' when transmission stopped. The expression later extended to television.