Equipment used for determining the direction from which a received signal is being radiated. Direction finders serve a variety of applications. In navigation, a direction finder on a moving vehicle, such as an aircraft or a ship can be used to provide either a transmitter's bearing to yield relative positioning information if the transmitter's location and range are known, or the vehicle's heading for guidance along a desired route toward a given destination. A direction finder may also be a ground station used to obtain bearing information on a moving source. This information, when passed on, can be useful for the moving source in navigating. Surveillance uses of direction finders include uncovering covert transmitters in espionage, locating the emitter in search-and-rescue missions, and tracking crewless objects in scientific missions.
Most direction-finding systems are based on two principles, one exploiting antenna gain characteristics and the other using phase-frequency relationships observed by a moving or a multiple-element antenna.
Direction-finding systems based on antenna gain sometimes take advantage of variations where the received signal amplitude is either a maximum or a minimum. Signal-minimum systems exploit the distinct null found in an antenna's gain pattern. Some of the earliest direction-finding equipment using loop antennas was conceived on this principle. The advent of higher-frequency systems and highly directive antennas, such as phased arrays and parabolic dishes, provided the basis for signal-maximum systems, which can pinpoint the direction in which the signal is strongest. See also Antenna (electromagnetism).
The principle of induced Doppler frequency modulation requires movement of the antenna to cause additional Doppler shift to the original signal. The frequency of the signal received by the moving antenna will vary in relation to that received by a fixed antenna. The antenna motion most suited for omnidirectional purposes is circular. For this, the induced Doppler is zero at the instant when the antenna motion is perpendicular to the bearing of the received signal. See also Doppler effect.
Airborne automatic direction finders (ADFs) are constrained by size and weight. For their simplicity, signal-minimum systems with loop antennas are widely used. See also Air navigation; Electronic navigation systems.
