Electrically Small Superdirective Endfire Arrays of Metamaterial-Inspired Low-Profile Monopoles

This letter presents the design and experimental characterization of subwavelength, superdirective, endfire antenna arrays operating in the 2.45-GHz band. Microstrip-fed, low-profile folded monopoles are designed using metamaterial-inspired phase-shifting lines composed of inductively coupled spiral resonators and employed for the synthesis of three different superdirective, two-element antenna arrays with variable interelement spacing. High directivity values are obtained experimentally. The experimental results show that the radiation efficiency of the reported arrays decreases as the interelement spacing of the arrays is reduced. Nevertheless, the directivity increase that is achieved with the interelement spacing reduction compensates the efficiency reduction, enabling even the more compact arrays to deliver reasonable gain values.