Experimental HF circular array with direction finding and null steering capabilities

The paper describes an HF-circular-array receiving and direction finding system which also offers a capability for null steering. The experimental array consists of four wideband active loop antennas which feed a 4 ¿¿ 4 Butler matrix. The action of the Butler matrix is to analyse the signals received by the array in terms of phase-mode components. Direction finding can then be carried out by measuring the phase difference between phase-mode outputs of the matrix, and either beam or null formation can be achieved by combining such phase modes with appropriate phase and amplitude weightings. The experimental system uses a microcomputer based adaptive technique for the bearing measurement. The advantage of this technique is that it employs a single unmodified HF receiver and it also offers a capability for null steering by means of software control of the receiving system.