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 audio tones with frequencies A and B results in an oscillation equal to frequency A minus B. This phenomenon is also exploited in the tuning of pianos.
Fessenden suggested that the heterodyne principle be employed in a radio receiver by mixing the incoming radio wave with a locally generated wave of slightly different frequency. The combined wave then drives the diaphragm of an earpiece at the frequency of the audio. For example, mixing a 101 kHz input with a 100 kHz signal 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. Little more than ten years later, however, electron tubes effectively filled the role of the oscillator and led to the construction of functional heterodyne receivers during World War I.
The basic scheme for superheterodyne radio receiver circuits of today was patented by Lucien Lévy (French patent or Brevet 493,660, filed 4 August 1917, granted 19 August 1919; and Brevet 506297, filed 1 October 1918, granted 27 May 1920), which he developed from Paul Laüt's earlier heterodyne technique. Edwin H. Armstrong reduced this to practice a year later, applying for U.S. patent 1,342,885 on 30 December 1918, which issued on 8 June 1920. Working in Germany, Walter Schottky also conceived of it independently (Deutsche Reich Patent 368,937, filed 19 June 1918, granted 12 February 1923). Many others--including John V. L. Hogan, Alexander Meissner, Harold J. Round, Georg von Arco, and Paul Laüt--made contributions.
Built on earlier heterodyne techniques, the essence of the superheterodyne circuit is to convert a high-frequency signal to one of intermediate frequency by heterodyning it with an oscillation generated in the receiver. The intermediate frequency signal is then amplified before the detection and amplification that usually occurs in receivers. The superheterodyne circuit has the ability to boost weak signals significantly and makes it possible to reduce the size of antennas dramatically.
While Lévy had applied superheterodyne technology for the purpose of encoding messages during the war, Armstrong exploited it first commercially to improve reception in a radio receiver. This became commercially important as the general public in the United States and western Europe began embracing broadcast news and entertainment in the early 1920s. RCA marketed the superheterodyne beginning in March 1924. It was more sensitive than the heterodyne receiver 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 circuit for radio receivers.
Alan Douglas, "Who Invented the Superheterodyne?" Proceedings of the Radio Club of America Vol. 64 no. 3 (November 1990), p. 123-42.
Edwin H. Armstrong Papers, 1886-1982, Finding Aid, Columbia University Libraries Archival Collections.
Walter H. Schottky, "On the Origin of the Super-Heterodyne Method," Proceedings of the I.R.E. 14 (October 1926), p. 695-8.