A sub-space radio was a device for communication between dimensions.
During his time working with UNIT, the Third Doctor had members of UNIT's staff aid him in constructing a sub-space radio so he could contact the inhabitants of a sub-atomic universe. He intercepted a distress call from the Antarians, and rigged a device which would transport him and Liz Shaw to that universe. After arriving there, getting captured by the Valerons, and escaping with the aid of Dynil, they were abruptly brought back to the Doctor's lab when the Brigadier, unaware of the adventure they were having, shut off the sub-space radio. (PROSE: A Universe Called Fred)