A FREE-SPACE BISTATIC CALIBRATION TECHNIQUE FOR THE MEASUREMENT OF PARALLEL AND PERPENDICULAR REFLECTION COEFFICIENTS OF PLANAR SAMPLES

A free-space bistatic measurement system suitable for operation in the frequency range 5.85-40 GHz is calibrated to measure the parallel and perpendicular reflection coefficients of metal-backed planar samples for obliquely incident waves. The measurement system consists of transmit and receive antennas in the bistatic configuration, mode transitions, precision coaxial cables, and the network analyzer. Diffraction effects at the edges of the sample are minimized by using spot-focusing horn lens antennas, which focus most of the energy on a one-wavelength diameter circular section of the sample. A new free-space bistatic calibration technique was developed to eliminate errors due to multiple reflections between transmit and receive antennas via the surface of the sample. The effect of defocusing due to the obliquely incident plane wave with focused antennas is minimized by introducing correction factors which modify measured reflection coefficients. Details of the calibration procedure and a discussion of the experimental results obtained for planar samples of Teflon and Eccogel 1365-90 in the frequency range 12.4-18 GHz are presented.