Heterodyne detector for measuring the characteristics of elliptically polarized microwaves

We present the design and results of an innovative device which is capable of determining the three characteristic parameters of elliptically polarized light (ellipticity, angle of ellipticity and direction of rotation) for microwave radiation at a frequency of 105 GHz. The device consists of two perpendicularly oriented rectangular pickup waveguides to measure the electrical field in two orthogonal directions. The microwave frequency is mixed down by a heterodyne technique to frequencies on the order of 200 MHz. An oscilloscope is used to determine the relative amplitudes of the electrical fields and the phase shift between them, from which the three characteristic parameters are calculated. As an essential part of the commissioning, the device was used to characterize the polarizers of the transmission line for the collective Thompson scattering (CTS) experiment at ASDEX Upgrade. The measurements were also used to benchmark the modeling of the microwave propagation through the transmission line. Comparison between measurements and calculated wave characteristics show good agreement and are presented.