A simple direction-finding scheme (A) and an ultrasonic direction finder (B)
A radio receiver, equipped with a signal-strength indicator and connected to a rotatable, directional antenna, can be used to determine the direction from which signals are coming. Radio direction find ing (RDF) equipment aboard a mobile vehicle facilitates determining the location of a transmitter. An RDF receiver can also be used to find one’s own position with respect to two or more transmitters operating on different frequencies.
In an RDF receiver for use at frequencies below about 300 MHz, a small loop or loopstick antenna is used. It is shielded against the electric component of radio waves, so it picks up only the magnetic part of the EM field. The loop is rotated until a sharp dip, or null, occurs in the received signal strength, indicating that the axis of the loop lies along a line toward the transmitter. When readings are taken from two or more locations separated by a sufficient distance, the transmitter can be pinpointed by finding the intersection point of the azimuth bearing lines on a map.
At frequencies above approximately 300 MHz, a directional transmitting and receiving antenna, such as a Yagi, quad, dish, or helical type, gives better results than a small loop. When such an antenna is employed for RDF, the azimuth bearing is indicated by a signal peak rather than by a null.