Editing Edwin Howard Armstrong (section)

===Superheterodyne circuit===

[[File:Edwin H Armstrong in WWI 1922.png|thumb|right|Armstrong in his Signal Corps uniform during [[World War I]]]]
The United States entered WWI in April 1917. Later that year Armstrong was commissioned as a captain in the [[Signal Corps (United States Army)|U.S. Army Signal Corps]], and assigned to a laboratory in Paris, France to help develop radio communication for the Allied war effort. He returned to the US in the autumn of 1919, after being promoted to the rank of Major.<ref name="Lessing-320" /> (During both world wars, Armstrong gave the US military free use of his patents.)

During this period, Armstrong's most significant accomplishment was the development of a "supersonic heterodyne" – soon shortened to "superheterodyne" – radio receiver circuit.<ref name="nyt19540202" /> This circuit made radio receivers more sensitive and selective and is used extensively today. The key feature of the superheterodyne approach is the mixing of the incoming radio signal with a locally generated, different frequency signal within a radio set. That circuit is called the mixer. The result is a fixed, unchanging intermediate frequency, or I.F. signal which is easily amplified and detected by following circuit stages. In 1919, Armstrong filed an application for a US patent of the superheterodyne circuit which was issued the next year. This patent was subsequently sold to Westinghouse.<ref>MacLaurin (1949), page 106. "Westinghouse then made an even more important move by purchasing [on May 22, 1920] for $335,000 the Armstrong patents on the regenerative and superheterodyne circuits."</ref> The patent was challenged, triggering another patent office interference hearing.<ref name="who">[http://antiqueradios.com/superhet/ "Who Invented the Superheterodyne?"] by Alan Douglas, originally published in ''The Legacies of Edwin Howard Armstrong'' from the "Proceedings of the Radio Club of America", Nov. 1990, Vol.64 no.3, pages 123-142. Page 139: "Lévy broadened his claims to purposely create an interference, by copying Armstrong's claims exactly. The Patent Office would then have to choose between the two inventors."</ref> Armstrong ultimately lost this patent battle; although the outcome was less controversial than that involving the regeneration proceedings.<ref>Lewis (1991), page 205. "...the case did not seem to affect Armstrong emotionally in the same way the regeneration suit did... Possibly he recognized the Frenchman did have some legitimate claim to the invention... Armstrong respected Levy in a way that he could not respect de Forest..."</ref>

The challenger was [[Lucien Lévy]] of France who had worked developing Allied radio communication during WWI. He had been awarded French patents in 1917 and 1918 that covered some of the same basic ideas used in Armstrong's superheterodyne receiver. AT&T, interested in radio development at this time, primarily for point-to-point extensions of its wired telephone exchanges, purchased the US rights to Lévy's patent and contested Armstrong's grant. The subsequent court reviews continued until 1928, when the District of Columbia Court of Appeals disallowed all nine claims of Armstrong's patent, assigning priority for seven of the claims to Lévy, and one each to [[Ernst Alexanderson]] of General Electric and Burton W. Kendall of [[Bell Laboratories]].<ref>''The Continuous Wave: Technology and American Radio, 1900-1932'' by Hugh G. J. Aitken, 1985, page 467.</ref>

Although most early radio receivers used regeneration Armstrong approached RCA's [[David Sarnoff]], whom he had known since giving a demonstration of his regeneration receiver in 1913, about the corporation offering superheterodynes as a superior offering to the general public.<ref>''History of Radio to 1926'' by Gleason L. Archer, 1938, [https://babel.hathitrust.org/cgi/pt?id=wu.89074767138&view=1up&seq=343 page 297]: "It appears that Armstrong first exhibited the device to the astute General Manager of RCA, David Sarnoff. Mr. Sarnoff had just concluded arrangements that involved ordering several millions' dollars worth of an improved type of radio... He was so impressed by the Armstrong invention that he at once halted these negotiations..."</ref> (The ongoing patent dispute was not a hindrance, because extensive cross-licensing agreements signed in 1920 and 1921 between RCA, Westinghouse and AT&T meant that Armstrong could freely use the Lévy patent.) Superheterodyne sets were initially thought to be prohibitively complicated and expensive as the initial designs required multiple tuning knobs and used nine vacuum tubes. In conjunction with RCA engineers, Armstrong developed a simpler, less costly design. RCA introduced its superheterodyne Radiola sets in the US market in early 1924, and they were an immediate success, dramatically increasing the corporation's profits. These sets were considered so valuable that RCA would not license the superheterodyne to other US companies until 1930.<ref name="who"/>