a type of direction finding in which the direction to a source of radio waves is determined by means of radio direction finders.

A radio direction finder consists of an antenna-feeder system, which is used to receive the radio waves propagating from the object on which a bearing is being taken, and a receiving indicator. Alternating electromotive forces are induced in the antenna-feeder system by the received waves, and, depending on the method employed, the amplitudes of the induced signals may be compared or the phase differences may be measured. This provides information regarding the angles between the direction to the object and given reference planes. In universal (two-coordinate) direction finders, both angles that determine the bearing are measured. In the azimuthal type, only one of the angles (the azimuth) is obtained; this method is employed in maritime navigation.

Depending on the extent to which the measurements are automated and the method of indicating the bearing, the following types of radio direction finders are recognized: the nonauto-matic (aural) type, with indications for the minimum or maximum audibility of the signals from the object; the semiautomatic (visual) type, with a pointer indicator or an oscilloscope display; and the automatic type, with a digital readout of the measured parameters.

Radio direction finding with two direction finders separated by a sufficiently great distance (such that the bearings to the radio source differ by no less than 30°) is capable of determining the position of an object from the intersection of the two bearings. Radio direction finding may also employ two or more sources of radio waves, for which bearings may be computed simultaneously or with short intervals between. This makes it possible to determine the position of the object from which the measurements are being made.

The phenomenon of directivity in radio reception, which is a characteristic of most antenna types and which is the basis of the amplitude method of radio direction finding, was noted by A. S. Popov. The invention of the loop antenna led to the development of the first radio direction finders. The Soviet scientists B. A. Vvedenskii, M. V. Shuleikin, and others have made important contributions to the theoretical and practical development of radio direction finding. The technique is employed extensively in maritime, air, and space navigation and in electronic reconnaissance, radio astronomy, and meteorology.