Measurement of cross-polar transmission in free space

A method is described for measuring the cross-polar transmission coefficient of a flat-panel sample in free space using a quasi-optical bench between 8 and 13 GHz. Impedance matching to both co- and cross-polar components simultaneously, so as to prevent reflection of one of them, from the rectangular waveguide feed, is provided by resistive vanes within the flares of the horn antennas. The bench is calibrated by means of a modified through-reflect-line (TRL) or through-short-delay (TSD) technique and uses a wire grid as a calculable reference standard. The calibration algorithm is based on Speciale's 'super-TSD' algorithm, which has been modified so that its stability is enhanced. The method is validated by comparison of measurements on wire grids with theoretical predictions, which showed excellent agreement, provided the test grid was accurately positioned with respect to the reference plane. The method was also compared with approximate experimental methods, which showed moderate agreement.