A surface unit for a sonobuoy or similar device is disclosed includes a variable volume sealed chamber (23) and an arrangement including a pressurized carbon dioxide cartridge (19) within the sealed chamber which, when energized, releases the carbon dioxide and increases the chamber volume from a collapsed volume to an expanded significantly greater volume. Electronic circuitry including radio transmitter circuitry is included within the sealed chamber along with a radio transmitter antenna which is within the sealed chamber and coupled to the radio transmitter circuitry. Expansion of the float sealed chamber functions to erect the antenna preparatory to transmission. The variable volume sealed chamber is formed of a semi-rigid plastic cup (32) having a generally cylindrical corrugated sidewall portion (31), a closed end describing one of the chamber ends and a rigid base member (25) sealed to the cup sidewall portion remote from the closed end and describing the other of the chamber ends. As it expands, cylinder length is directly proportional to cylinder volume.