How did radio direction finders become useful for aircraft navigation?

Radio direction finders (DF) became essential for aircraft navigation by allowing pilots to determine their position relative to known radio transmitters, even in conditions of poor visibility such as fog or darkness.12

The system functions by using a loop antenna to detect the direction of a radio signal. Because the signal is weakest when the plane of the loop is perpendicular to the incoming radio waves, a navigator can rotate the antenna to find this "null" point, thereby identifying the bearing to the transmitter. By obtaining bearings from two or more known stations, a navigator can use triangulation to fix the aircraft's precise location.132

As aviation technology grew, airborne direction finders—often called radio compasses—allowed pilots to take continuous bearings on shore beacons or other receivable signals. This development provided a significant advantage over earlier systems that relied on ground stations to calculate a craft's position and relay it back, a process that was often overloaded during bad weather. Eventually, these technologies evolved into more sophisticated radio-beam systems and VOR beacons, which provided more automated and accurate navigational guidance.14