Challenges and Limits of Designing Wideband and Efficient Compact Superdirective Antenna

The design of an efficient and a wideband superdirective array is a challenging and a difficult task. The challenges lie in the inverse proportionality between the maximum achievable directivity in one hand and the efficiency as well as the impedance bandwidth in the other hand. In this paper we present a simple optimization strategy to address these issues. It consists of two key elements: the embedding of a set of passive loads inside the antenna structure to ensure a wideband behavior and the optimization of the array realized gain instead of its directivity. This is equivalent to a multi-objective optimization of the array directivity, its radiation efficiency as well as its return loss. Such optimization allows to establish a best trade-off between the array performances. The proposed method is used afterwards for designing a two-element array with a separation distance of $d=0.1\lambda$ . The simulations show that the array presents a wide impedance bandwidth of 11.8%, a large -1 dB bandwidth of directivity and gain of 18.2% and 13.6% respectively and a high total efficiency > 80 %.