Design of Coplanar Sensor for the Permittivity Measurement of Thin Dielectric Samples

This paper explores the nondestructive permittivity measurement of thin dielectric samples using the designed coplanar waveguide sensor. A conductor backed coplanar line is designed for 50 ohms, and the sample whose permittivity is to be measured is placed freely on top of this sensor. This structure of the coplanar sensor with sample placed on it is referred as MCCPW (multilayered conductor backed coplanar waveguide), and the scattering parameters of the MCCPW structure are measured using the VNA (Vector network analyzer). To reduce effect of higher order modes, the sample is placed at the center of the coplanar sensor. From measured scattering parameters, the effective permittivity of the MCCPW structure loaded with the test sample is calculated using the proposed reflection-transmission approach. The fill factors are calculated from the geometry of the MCCPW structure. In this paper, instead of using the costly microwave probe stations for the measurement of scattering parameters of coplanar lines, two SMA to coplanar end launchers are used. A simple de-embedding technique based on the transmission matrix method is used to extract the scattering parameters at the coplanar sample interface. Two coplanar sensors with length 25.4 mm and 50 mm are designed and fabricated. The designed sensor is tested by extracting the permittivity of a number of reference samples from both the simulated and the experimental data.