Interested from childhood in anything mechanical, as an engineer, Armstrong constructed a variety of wireless equipment in the attic of his parent’s home, including spark-gap transmitters and receivers with iron-filling ‘coherer’ detectors capable of producing weak Morse-code signals.
In 1912, while still a student of engineering at Columbia University, he applied positive, i.e. regenenarative feedback to a Lee De Forests triode amplifier to just below the point of oscillation and obtained a gain of some 1,000 times, giving a receiver sensitivity very much greater than hitherto possible.
Furthermore, by allowing the circuit to go into full oscillation he found he could generate stable continuous-waves, making possible the first reliable CW transmitter.
Sadly, his claim to priority with this invention, for which he filed US patents in 1913, the year he graduated from Columbia, led many to many years of litigation with De Forest, to whom the US Supreme Court, but unjustly awarded the patent in 1934.
During the First World War, after sometime as an instructor at Columbia University, he joined the US Signal Corps laboratories in Paris, where in 1918 invented the superheterodyne, a major contribution to radio-receiver design and for which he filed patent in 1920.
The principle of this circuit which underlies virtually all modern radio, TV and radar reception, is that by using a local oscillator to convert, or “heterodyne”, a wanted signal to a lower, foxed, ‘intermediate’ frequently it is possible to obtain high implication and selectivity without the need to ‘track’ the tuning of numerous variable circuits.
Returning to Columbia after the war eventually becoming Professor of Electrical Engineering, he made a fortune from the sale of his patent rights and used part of his wealth to fund his own research into further problems in radio communication particularly that of receiver noise.
In 1933, he filed four patents covering the use of wide-band frequency modulation (FM) to achieve low noise, high fidelity sound broadcasting, but unable to interest RCA he eventually built a complete broadcast transmitter at his own expense in 1939 to prove the advantages of his system.
Unfortunately, there followed another long battle to protect and exploit his patents and exhausted and virtually ruined he took his own life in 1954, just as the use of FM became an established technique.
Armstrong, Edwin Howard
