 Radio direction finding

Radio direction finding estimates the direction from which a radio wave arrives at a receiving station. It uses directional information in an antenna response, such as changes in amplitude or relative phase across several antennas. A direction is a bearing from one station; it is not, by itself, the distance to the transmitter. Combining bearings from separated stations can constrain position.

A two-element interferometer illustrates the measurement. For a plane radio wave arriving at angle $\theta$ from the perpendicular to an antenna baseline of length $d$, the path difference is $d\sin \theta$. Thus the relative phase iswhere $\lambda$ is the wavelength. With $d=\lambda /2$ and measured $\Delta \varphi =\pi /2$, the ideal model gives $\sin \theta =1/2$, hence $\theta =30^{\circ }$ within a chosen arrival sector. One baseline does not resolve every directional ambiguity, and phase wrapping can introduce additional candidates. More antennas and other directional information can help distinguish them.

A useful field test places a transmitter at a known location, compares measured and geometric bearings, and repeats the comparison across directions and frequencies. The International Telecommunication Union's originating test recommendation includes controlled transmitters and known broadcast stations, emphasizing that a final installation can behave differently from an ideal test range. Reflections, obstructions and signal properties can bias the direction estimate; an error measured at one site cannot simply be transferred to another installation.^[ref]