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 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 one of the radio sets that are kept in a room inside the tunnels and a sub that is stationed somewhere in the English Channel.
The radio transmits and receives via an antenna that is kept inside of the flag pole that is set atop of Klink's office and is raised using a lever in the radio room in the tunnels. In one episode, a new cross-shaped antenna was necessary, but was thought to be too obvious and get noticed, so Colonel Hogan bent the arms of the antenna into a swastika.