An improved apparatus and method for radio direction finding is set forth, the generic function of which is that of an "automatic bearing indicator" (ABI). The technique and apparatus are intended for use in association with an antenna system which provides two directional beams which are scanned azimuthally in synchronism and are accessible simultaneously at two separate ports. The first beam is characterized by a response pattern having two adjacent, equal, lobes separated by a single, simple, null. The second beam is characterized by a response pattern having a single lobe, aligned in angle with the null of the first beam. The two signals from the two ports of the antenna system are directed via a twin channel receiving means, phase difference and amplitude detection means, and analog to digital conversion means, to data processing means under control of an algorithm which exploits the properties of the phasor (complex number) ratio of the first signal to the second signal. Determination of the azimuthal direction of the incident radio wave is thereby enabled 1) with fewer scanning cycles (typically one), 2) with greater independence from any intentional modulation on the signal, and 3) with improved accuracy in the presence of multi-component (multi-path) incident radio wave fields.
Edgar C. Hayden