Calibration techniques for free space reflection coefficient measurements

Owing to the calibration standards and techniques, free space microwave reflection coefficient measurements, commonly used for characterising RAM radar absorbing materials reflectivity and material parameters, are prone to significantly more errors than those made in coaxial lines or waveguide. The most commonly used free space calibration procedure using an empty room and a metal plate is described, and the errors arising critically examined. Alternative calibration procedures using both two and three term error correction are investigated both theoretically and experimentally and shown to be efficacious in reducing measurement errors still further. The authors discuss the need for a free space reflectivity standard, thus enabling the verification of free space reflection coefficient measurement systems to be achieved.