DIRECTION FINDER

radio receiver and antenna system for determining the direction of the source of a signal. A direction finder may be a ground station that takes bearings on ships or aircraft, a device used for military or investigative purposes, or an airborne or shipborne navigational device, also called a radio compass. The directional property of a radio antenna that is formed into a coil or loop was familiar to the earliest experimenters. As a loop is rotated horizontally its energy output, derived from the interception of a radio wave, passes through a sharp minimum that corresponds to the direction of the radio transmitter. Soon after ships were first equipped with radio, shore direction-finding (DF) stations were placed at strategic points along navigational routes and near harbour approaches. Upon receiving a request by radio from a ship, two or more shore stations determined the directions from which the ship's signal arrived by means of their loop antennas and transmitted this information back to the vessel. However, this service was limited to one vessel at a time, a serious drawback in bad weather, when demands were heavy. By reversing the processplacing the transmitter ashore and the direction finder on the shipthe system became nonsaturable and the navigator was given two further advantages: he was able to take continuous or frequent bearings on any shore beacon, and he could take bearings of any receivable signal, such as transmissions from broadcasting stations and from other vessels. This change in the system was roughly coincident with the initial growth of aviation, and the airborne direction finder immediately became a valuable aid to air navigation. Crossed-loop direction finders, used when it is not convenient to rotate the loop, consist of two loops mounted at right angles to each other with their terminals connected by cable to two crossed fixed coils of a goniometer; a rotatable coil in the goniometer simulates the rotation of the ordinary loop antenna. Other types of DF antennas include the Adcock (using spaced dipoles or monopoles) and spaced-loop types that have been employed ashore as navigational aids in the high-frequency and very-high-frequency bands, where greater efficiency and accuracy is obtained than with the simple loop. Loops incorporating ferromagnetic cores are largely used in portable direction finders, produced principally for aircraft and small boats.