An adaptive circular array for HF direction finding and null steering

A HF direction finding (DF) system based on a four-element array of active loop antennas is analyzed. DF can be performed by observing the phase differences between the excited phase modes. The excitation of the array in its phase mode is studied using a Butler matrix. Adaptive processing is applied to measure the relative phases of the phase modes and to estimate the incident signal bearing. The use of a single unmodified receiver for frequency tuning the DF system is examined. The phase shifter, phase perturbator, electronic attenuators, receiver, and microcomputer used to experimentally test the DF system concept are described. The benefits the microcomputer and null steering capabilities provide for the system are discussed. The DF system test data reveal a DF accuracy of 3 degrees.