(b) Early type of radiotelephony equipment using valve type E, made in France circa 1917.

After the WWI the radiotelephony was wide used in aircraft communication, which adoped some techniques originally developed for the navy since 1914.For instance, the use of the code of 3 letters, as known as the Q code developed by the International Telecommunication Union. (a)
In the later 1930's due to the continuous development of the electronics technology, many improvements were introduced in the radio communication equipments either in circuitry as in desing. In this way radio receivers were already supplied with the new superheterodyne circuit topology, for operation in amplitude modulation in the frequency range from 1,9 to 3 m or 100 to 156 MHz, divided in several channels. The transceivers for aeronautical applications were sturdy and compact provided with innovations as the SQUELCH circuit developed to suppress the carrier noise. (b)