In 1904, Valdemar Poulsen, the same Danish scientist who invented the magnetic recording process did the first transmission of voice by the radio, through the telegrafone, an invention based originally in the telephone. (a) However, the communications through the radio had only been possible with the invention and the improvement of the thermionic valve.
Thus, its application in the military communication is related with some technological developments as:

- The valves used as repeater in telephony
- The metallic valves
- The miniature valves
- Valves for high frequency.