Seventy-five years ago this month, the April 1942 issue of Radio Craft showed the defense-minded electronics hobbyist or serviceman how to put together this aircraft detector. According to the magazine, the detector would be of particular interest to civilian defense units around the country. Cost of parts was set at $50, and the detector used readily available parts. In tests, it was able to pick up conversations at three blocks, and the sound of a bomber ten miles away.
The “ear” consisted of an old phonograph horn with a sensitive microphone mounted at the base. The four-tube battery operated amplifier employed three 1H5GT and one 1G4G tubes and allowed the operator to scan the skies for approaching planes with headphones.