W-Band Sensor for Complex Permittivity Measurements of Rod Shaped Samples

Abstract

A novel W-band sensor for complex permittivity measurements of rod shaped samples based on reflectometer method is described. Characterization of dielectric materials around 100 GHz is quite a challenge. The main limitation is the requirements for the accuracy of the dimensions of the test sample and ensuring good contact with conductive surfaces. From this point of view, it seems interesting to use a cylindrical waveguide with the TE01 mode. The four linearly-polarized H10 fields of rectangular waveguides with equal amplitude and phase circumnavigate the cylindrical waveguide to jointly excite its TEnm circular modes with four-fold symmetry. This precludes the excitation of lower-order modes of the metallic cylindrical waveguide. TE01 mode is unique in that its field leads to zero on the surface of the walls of a metallic cylindrical waveguide. The main field interaction takes place on the sample surface. The calibration method uses calibration curves obtained by simulation for known values of complex permittivity of the samples. This eliminates the requirement for a precision calibration standards, which is the problematic to realize at millimeter wavelengths. Moreover, the complex field structure in the sensor makes it impossible to apply a standard calibration method. Simulation and measurements using these method have been performed in W-band (70–114 GHz). Measurement results demonstrate the robustness of this new sensor for characterizing rod shaped dielectric samples.