On the Layout Paradox of Super-Directive Antenna Arrays

Super-directive antenna arrays (SDAAs), which rely on exploiting mutual coupling as a design parameter, have attracted considerable attention in recent years due to their ability to achieve high directivity within electrically small footprints. In many studies, extremely high directivity values are targeted by reducing the inter-element spacing and thereby strengthening mutual coupling. However, the realized gain is often overlooked in this process, which leads to a practical array layout paradox: configurations that theoretically promise higher directivity do not necessarily provide improved realized gain in practice. In this study, the aforementioned paradox is investigated by examining the feasibility of achieving super-directivity under different physical layout constraints. By imposing practical limits on the array geometry, the relationship between array compactness and achievable realized gain is systematically analyzed. A particle swarm optimization (PSO) algorithm is employed to determine the excitation coefficients that maximize the realized gain for various array configurations, and the convergence behavior of the optimization process is evaluated. In addition, the computational performance of the proposed optimization framework is compared with full-wave simulations. The results show that the developed approach can provide solutions consistent with full-wave analyses while requiring significantly less computational time.