The increasing roleof single-seater aircraft in WWI put a premium on direct speech transmission, forcing the development of equipment based upon electron tubes (or "valves") which alone could provide it. This wireless telephone from Marconi's Aircraft Division is dated 1921. Radio telephony communications to, from and between aircraft were "in clear" i.e., not encoded, an exception to general military practice. The international standard aircraft frequency. post WWI, was 330 kHz. There was an alternative emergency frequency of 500 kHz. Maximum permissible power was 700 watts. With this equipment, Marconi guaranteed a range of 100 miles.
From T. Devereux, Messenger God of Battle, Brassey's, London, 1991.