A free-space reflectometer for surface impedance measurement of materials in the Ku-band

This paper presents a free-space reflectometer that has been developed to measure surface impedance of planar materials in the Ku-band (12.5 to 18 GHz). The set-up uses a microwave lens made of metal plates to create an equiphase wavefront arriving at normal incidence on the sample surface. Thus, the TEM wave assumption can be made to extract the impedance of the sample, which is not the case for other free-space methods using lenses. It also has the advantage of requiring small sample size (17 cm/spl times/17 cm). The system has been validated with plexiglass. The maximal error on the reflection coefficient was 0.02 for the amplitude over the 5.5 GHz bandwidth. The accuracy of the system makes it suitable for shielding effectiveness measurement and characterization of conductive polymers or thin metallic layers.