In radio broadcasting, a transmitting antenna sends out electromagnetic waves that carry the radio program. A radio antenna may pick up these electromagnetic waves. The free electrons in the metal antenna are jostled back and forth by the passing radio wave. Converting the tiny currents created by this jostling into the speech or music of the radio program is the job of a radio receiver.
There are many different ways a radio receiver can perform this conversion. Reginald Fessenden was the first to apply one of these methods, called the heterodyne principle, to wireless communications in 1901. Fessenden coined the term heterodyne from the Greek words for other and force. The heterodyne principle is based on a well-known sound phenomenon where the combination of two different audio tones of frequencies A and B results in an oscillation with the frequency A minus B. This phenomenon is exploited every day in the tuning of pianos.
Fessenden suggested that the heterodyne principle be employed in a radio receiver by mixing the incoming radio-frequency wave with a locally generated wave of slightly different frequency. The combined wave then would drive the diaphragm of an earpiece at the frequency of the audio. For example, mixing a 101 kHz input with 100 kHz generated by the receiver yields a frequency of 1 kHz, which is in the audible range. Lacking an effective, inexpensive local oscillator, Fessenden could not make a practical success of the heterodyne receiver. Later, however, electron tubes effectively filled the role of the oscillator and led to the building of functional heterodyne receivers during the World War I.
The basic scheme for radio receivers of today was invented by Edwin H. Armstrong in 1918. Building on Fessenden’s earlier heterodyne technique, Armstrong's scheme is known as the superheterodyne method, and its ability to boost weak signals made it possible to reduce the size of antennas dramatically. Armstrong applied for a patent on 30 December 1918, which was issued on 8 June 1920. Armstrong exploited the heterodyne principle in a different way in his superheterodyne receiver, however. The essential idea was to convert the high-frequency signal to one of intermediate frequency by heterodyning it with an oscillation generated in the receiver. The intermediate-frequency signal was then amplified before the detection and amplification that usually occurs in receivers. Armstrong shared the credit for the superheterodyne with others who worked largely independently. Of special note were Lucien Lévy in France and Walter Schottky in Germany. RCA marketed the superheterodyne beginning in March 1924. It was more sensitive than the heterodyne and could be tuned by turning a single knob. Not long after RCA began licensing other manufacturers to make the superheterodyne, it became the standard type of radio receiver.
Armstrong went on to other achievements. In 1922, he introduced the super-regenerative circuit, which was widely used in special-purpose receivers, such as police radios, ship-to-shore radios, and radar systems. In the late 1920s and early 1930s, he almost single-handedly developed wide-band FM technology.
There were very strong links between Armstrong and Bill Eitel of Eimac, with an archive material located at Columbia University.