DF to ADF
The basic concept is that a loop antenna oriented perpendicular to the incoming radio wave will get an induced current flow clockwise in one half and counter-clockwise in the other half, thus producing a "null" signal. Turning the antenna 90 degrees will get a maximum signal. The "A" in ADF came along when they got the radio to do this automatically instead of manually, solve the pesky "180 degree ambiguity", and provide the result on a compass card.