Comment on "A Quasi-Optical Isolator" [Comments]

In the above named letter, the authors describe an interesting design of a linear-to-circular polarizer. Their use of six cascaded printed gratings is indeed a practical way to realize this type of quasioptical isolator. Their use of a resistor loaded grid is a good way to absorb the reflected wave. In this comment it is pointed out here that there have been earlier demonstrations of nonferrite quasioptical isolators. The use of a linear-to-circular polarization transducer to isolate the input port from reflections from the output and to do so in quasioptics dates from the mid-1950s. Millimeter and submillimeter versions were demonstrated in the 1960s. The basic idea is briefly described in a 1962 text by M.I. Skolnik.