I. Introduction
A retrodirective array (RDA) is a special type of phased array antenna that sends a signal back to the location of the interrogator without prior knowledge of the angle of incidence. Compared to smart antennas, whose beam control is based on digital signal processing, RDA systems are much simpler and possibly faster because no digital computation is required. The required phase relationship can be obtained by an analog circuit. Although the formation of the retrodirective beam is limited by being directed only to the source radiator, this is achieved without knowledge of the source location. Its simplicity allows the array to track fast moving sources [2]–[5]. It has the properties of high link gain and low cost that make the RDA useful in many fields, including wireless communication systems [6]–[7], automatic pointing and tracking [8], phase aberration correction [9], and phase conjugate resonators [10].