Theoretical and experimental properties of two-element, multiplicative multi-frequency receiving arrays including superdirectivity

The paper presents the theory and properties of the directional responses of two-element multiplicative arrays, sometimes termed correlation arrays, when receiving multi-frequency signals. The results of an experimental investigation employing band-pass signals are given and agree well with the predicted directional responses. Interesting modifications to the straightforward multi-frequency arrays are demonstrated, including a split-beam response, arrays with one channel clipped, and superdirective arrays. Superdirectivity is examined in terms of the improved effective aperture, for which a quantitative definition is proposed. Some degree of multi-frequency superdirectivity was obtained, but further gain of effective aperture is restricted by the accuracy to which the amplitudes of the received frequencies may be controlled.