 During World War II, Americans, Britains, Frenchmen, and other civilians
were seriously engaged in helping to defend their homeland. Those who were not in
the military gathered bottles, tin cans, tires, and clothing to use in the war effort.
Others volunteered at the Red Cross, veterans' hospitals, and USO offices. Some
stood guard at their nation's seashores and land borders, both as armed sentries
and as troop and aircraft spotters. As part of the civil defense effort, listening
devices were built to help detect the sound of approaching airplanes.
 In patriotic tradition, magazines like Popular
Science published many articles to assist the population contribute. Here
is a plan for a "Homemade Plane Detector." It used a horn "antenna" that fed into
a 3-stage tube-based (1H5GT) amplifier + 1G4G headset driver circuit. You could
build and power one today with a couple 1.5 V batteries - oh, and a 90 V and a 135
V battery.
See also
Aircraft Detection Prior to the Invention of Radar
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