On the realization of superdirectivity using passive scatterers

The problem of scattering of electromagnetic waves from cavity-backed apertures (CBAs) has received considerable attention. Single elements with resonant cavities have been investigated by a number of authors. A thin, cylindrical conducting screen with a longitudinal slot is a simple case of a two-dimensional CBA. An efficient solution to the problem of scattering from the slotted cylinder with a circular cross-section has obtained by using a rigorous approach based on the solution to the Riemann-Hilbert problem in the theory of functions of complex variables. The paper shows that the occurrence of the superdirectivity can be attributed to the excitation of modes that induce a phase reversal in the adjacent scatterers, and that the amplitude distribution also plays an important role. It demonstrates that a moderately high superdirective antenna can be achieved by using passive scatterers formed by resonant elements. It shows how the mutual coupling between the elements can be exploited to generate and control the desired phase and amplitude behavior of the near field that lead, in turn, to the superdirectivity in the radiated field. Since all the elements of the scattering array are entirely passive, it is somewhat remarkable that such a structure would generate a superdirective behavior. Representative numerical results are presented to illustrate the performance of superdirective arrays.