English: The first amateur superheterodyne receiver, an illustration for a do-it-yourself article in a 1920 amateur radio magazine. The superheterodyne circuit on which virtually all modern receivers are based was invented in 1918 during World War 1 by Edwin Armstrong when he was a captain in the U.S. Signal Corps, part of a secret project to eavesdrop on German radio communications. Paul F. Godley (in photo), a radio amateur and receiver expert for American Marconi during the war, heard about it and built this homebrew version (Armstrong's paper on the superheterodyne hadn't even been published yet) Described in companion article Paul F. Godley, "High Amplification at Short Wave Lengths," The Wireless Age, February 1920, p. 11-14, it is a 9-tube superheterodyne using Western Electric VT triodes. One tube is the mixer, which has regeneration to improve the selectivity; one is the local oscillator; there are 5 RC-coupled stages of IF amplification (mounted on the vertical board), the last of which serves as a detector; and 2 transformer-coupled audio amplifier stages. It uses an intermediate frequency (IF) of around 50 kHz. The attraction of the new superheterodyne circuit, as indicated by the article title, was that it could reach higher frequencies than existing receivers, up to the 200 meter (1.5 MHz) shortwave band, and was also more sensitive. The circuit was so new that the word "superheterodyne" doesn't even appear in the article.