Small Antenna Handbook

Chapter 6

Superdirective Antennas

6.1 History and Motivation

A useful operational definition of antenna array superdirectivity (formerly called supergain) is directivity higher than that obtained with the same array length and elements uniformly excited (constant amplitude and linear phase). Superdirectivity applies in principle to ESA, to apertures, to arrays of isotropic elements, and to actual antenna arrays composed of nonisotropic elements such as dipoles, slots, and patches. Small dipoles and loops are superdirective; their directivity remains at 1.5 as size decreases, but their efficiency decreases. Excessive array superdirectivity inflicts major problems in low radiation resistance (hence low efficiency), sensitive excitation and position tolerances, and narrow bandwidth. It is important to distinguish between directivity and gain. Gain as used in this book follows the antenna industry definition: Gain includes the effects of both losses (conductor and dielectric) and impedance mismatch. The IEEE and textbook definition where only loss is included is unrealistic and of little use; directivity includes neither loss nor impedance match. Thus, the gain of a superdirective antenna may be low.

Taylor (1948) was one of the first to use the term “superdirectivity”; supergain should include efficiency, which in many cases would negate the directivity increase. He proposed a physical explanation of superdirectivity in terms of spherical modes and their cutoff due to dimensions. ...