By the rear B-25 crew hatch, an ADF Antenna loop/control LP-21-A (large black "teardrop" used on B-17, B-24, B-25, C-47, otherWW II multiengined aircraft). Corrected 07-06-1944. An automatic direction finder (ADF) is a marine or aircraft radio-navigation instrument that automatically and continuously displays the relative bearing from the ship or aircraft to a suitable radio station.