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.
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.
Aircraft Detection Prior
to the Invention of Radar