Reflection Measurement of Low-Reflective Loads at Millimeter Waves

The well-known cross-ratio method of correction of the complex reflection coefficient of low-reflective waveguide one-port devices, which is well established up to 110 GHz, is extended to millimeter wave frequencies where the impedance standard, a quarter-wavelength rectangular waveguide section at midband, may become too thin to be fabricated and handled. Therefore, an alternative extended cross-ratio correction method (ECM) is presented where a pair of longer waveguide sections having well-defined cross-sectional dimensions and lengths is used instead. Their difference in length is a quarter-wavelength at midband. The method is tested in the WR-5 waveguide band (frequency f = 140-220 GHz), where a quarter-wave line is still thick enough to handle. The measurement results obtained by the ECM using two lines agree well with the results from the single-line cross-ratio method as well as with measurements obtained from a line-reflect-line calibration. Sensitivity coefficients are derived, which indicate how the measuring result is influenced by deviations of the different input parameters that are necessary to calculate the reflection coefficient.