Fessenden, Reginald Aubrey (1866–1932)

Canadian physicist who worked in the USA. He patented the modulation of radio waves (transmission of a signal using a carrier wave), an essential technique for voice transmission. At the time of his death, he held 500 patents.

Early radio communications relied on telegraphy by using bursts of single-frequency signals in Morse code. In 1900 Fessenden devised a method of making audio-frequency speech (or music) signals modulate the amplitude of a transmitted radio-frequency carrier wave – the basis of AM radio broadcasting.

Fessenden's other major invention was the heterodyne effect. In this, the received radio wave is combined with a wave of frequency slightly different to that of the carrier wave. The resulting intermediate frequency wave is easier to amplify before being demodulated to generate the original sound wave.

Fessenden was born in East Bolton, Québec, and educated at Bishop's University, Lennoxville, Québec. He went to the USA to work for inventors Thomas Edison and, 1890–92, George Westinghouse. Fessenden became professor of electrical engineering at Purdue University, Lafayette, and then at the Western University of Pennsylvania (now the University of Pittsburgh). It was there that Fessenden began major work on the problems of radio communication.

In 1902 Fessenden organized the building of a 50-kHz alternator for radiotelephony by the General Electric Company. This was followed by his building a transmitting station at Brant Rock, Massachussetts. On Christmas Eve 1906, the first amplitude-modulated radio message was broadcast.