I. Introduction
IN RECENT years, many RF systems have been designed with directed beam-forming capabilities for military and civilian communication applications. These systems are located on mobile platforms such as land vehicles, aircraft, and satellites. In addition, the platforms are operating in congesting environments that require multiple frequencies, code-division multiple access, or directed RF transmission in order to isolate the signals. In the case of directed transmission, phase shifters are used in order to perform narrow beam tracking. However, another technique exists. Developed circa 1950 [1]–[6] and performing the essential functions necessary for a directed RF link, it is called phase conjugation and has been used by many in the optics field for adaptive reconstruction of highly distorted optical signals [7] and, more recently, for RF communications [8], [9]. Phase conjugation at microwave and millimeter-wave frequencies is implemented using mixers as a substitute for materials with innate mixing properties, such as are found at optical frequencies [10].