A quasi-optical isolator was designed to avoid stability problems of heterodyne detection receivers arising from time variable standing waves in radio telescopes. These effects cause serious distortions of the spectrum because they are proportional to the field, while the detected power is dependent on the field square . In addition to the investigation of the isolator performance the origin of the standing-wave effects in heterodyne-detection receivers was examined measuring their suppression.
K. Eigler, K. Eigler,
"A tunable quasi-optical isolator", Proc. SPIE 1576, 16th International Conference on Infrared and Millimeter Waves, 15764Y (1 October 1991); doi: 10.1117/12.2297891; https://doi.org/10.1117/12.2297891