Electrically small superdirective arrays using parasitic elements

Electrically small superdirective arrays that are theoretically possible have been shown feasible for two and three element arrays. However the individual elements must be fed separately with the correct input current magnitudes and phases to achieve superdirectivity. In this paper, almost equivalent superdirectivity was obtained with two-element arrays by feeding only one element and shorting the second "parasitic" element. This parasitic array exhibits a new resonant frequency f_a0 which is usually lower than the single-element resonance, and two "superdirective" frequencies emerge on either side of f_a0 at which the parasitic element acts either as a director or a reflector. It was found, in general, that parasitic superdirectivity is optimized for electrically small elements by using the parasitic element as a reflector and by choosing the correct element separation and element orientation (for asymmetrical elements). In addition, the use of relatively low Q elements allows for smaller separation distances, resulting in increased directivity while achieving wider bandwidths. A brief overview of superdirective arrays was presented and separately fed and parasitic two-element superdirective arrays were compared