Radio

From Hogan's Heroes

The radio is used by the prisoners to keep in contact with their controllers in London, as well as the members of the local underground. The radio, which is most likely a wireless set, would be used to receive orders from London and send back communications to same, sometimes by voice and sometimes by the use of morse code. Radio contact is made via a receiver that is kept inside the ball of the flag pole that is set atop of Klink's office and a sub that is stationed somewhere in the English Channel.

The radio is kept in a room inside the tunnels.

There are several episodes in the series in which the Germans would try to locate the radio that is being used to contact the underground, not knowing that it is actually under their feet.