I. Introduction
Retrodirective arrays (RDA) have the unique ability to transmit a received signal back toward the interrogator without prior knowledge of the interrogator's location. Under a dynamic communication link where either the interrogator or the responder is in motion, the responder equipped with the RDA can track the interrogator, thereby maintaining a more stable communication link. This concept may be applied to remote sensor, identification tags, unmanned space crafts, etc. There are different techniques to achieve this functionality of rescattering the signal back to the interrogator. However, when high link gain and high-speed target tracking is desired, analog self-phasing method has been the candidate of the choice [1]. The invention of the Van Atta array in 1955 paved the notion of creating analog RDA systems in the form of an array [2]. In addition, unlike the traditional corner reflector, Van Atta arrays enabled the devices to be built entirely on planar substrates with a wider retrodirective angle [3]. Novel techniques and devices based on the Van Atta concept are actively being studied [4]–[6].