Design of a Super Directive Four-Element Compact Antenna Array Using Spherical Wave Expansion

In this paper, a super directive four-element compact parasitic antenna array has been designed and optimized using an ad hoc procedure based on the Harrington maximum directivity definition and spherical wave expansion. The complete design procedure (i.e., calculation of the optimum excitation coefficients and equivalent impedance loads associated to each parasitic element) is presented and validated through full-wave electromagnetic simulations. The proposed array has been designed at 868 MHz; its electrical size is 0.45λ_o × 0.36λ_o (diameter of minimum sphere circumscribing the antenna equal to 0.57λ_o). A prototype has been built and tested to demonstrate the effectiveness of this design. A maximum directivity of 11.7 dBi has been measured at 871 MHz in satisfactory agreement with theoretical results and electromagnetic simulations.