A Millimeter Wave Complex Reflection Coeficient Scanner

A broadband microwave measurement system has been designed to make complex reflection coefficient measurements on a swept basis at millineter wave frequencies. This instrument covers a frequency range of 50-75 GHz in WR-15 rectangular waveguide. Two appliques are being added in WR-22 and WR-10 waveguide to extend the frequency range of the system to 33-110 GHz. Measurements can be made while sweeping over bands as wide as 10 GHz. An interesting feature of the complex reflection coefficient scanner is that residuals, or baseline, of the system are automatically subtracted. The resultant output display is a real-time polar plot of the actual reflection coefficient of the network under test. Residuals of 0.03 for reflection coefficients around unity, and 0.013 for reflection coefficients around zero can be measured with a resolution of 0.01 while sweeping over a 5 GHz band. A maximum reflection coefficient range of 46 dB has been achieved using straightforward video detection techniques. For measurements of networks with very low reflection coefficients (0.01 and lower) a time averaging feature is available to reduce the effects of random noise. The complex reflection coefficient of the unknown can be displayed on an oscilioscope and photographed, or read out on an X-Y plotter.