2-18 GHz circular array interferometer DF antenna system

The DF antenna system contains two 3:1 bandwidth circular array interferometers (CAI). Each CAI consists of an 8-element circular-array aperture coaxially connected to an 8-input, 4-output Butler matrix mode-forming network as shown. The Butler matrix outputs represent the omnidirectional phase modes of the CAI. High-accuracy (1.2 deg. RMS), monopulse angle-of-arrival (AOA) measurement over a full 360-degree instantaneous field of view is obtained using only three receiver channels. The CAIs provide true azimuth AOA independent of the elevation angle; unlike linear interferometers, which exhibit coning about their baseline. Of the remaining Butler matrix outputs, one is utilized as a wide-open signal acquisition channel and the other four are terminated. Using a common aperture with a 360 degree instantaneous field of view enables acquisition and AOA determination to be performed simultaneously on every pulse. The common aperture also eliminates differential multipath between the acquisition and DF channels. In addition, it also minimizes the antenna size and overall system cost.