foxhole radios consisted of a wire aerial, a coil of wire serving as inductor, headphones, and some sort of improvised diode detector to rectify the signal. detectors consisted of an electrical contact between two different conductors with a semiconducting film of corrosion between them. they were devised from various common objects. one common type was made from an oxidized razor blade (rusty or flamed) with a pencil lead pressed against the blade with a safety pin. the oxide layer on the blade and the point contact of the pencil lead form a semiconductor schottky diode and only allow current to pass in one direction https://en.m.wikipedia.org/wiki/foxhole_radio