Microwave permittivity measurements through cross-shaped ring sensors

A microwave sensor designed to determine the complex permittivity of planar materials is proposed. The sensor is implemented using a full planar microstrip technology. The near fields of the ring resonator interact with the material under test (MUT) and the electrical characteristics of the resonator change. A scalar network analyzer is used to measure the resonant frequency and the corresponding amplitude of the transmission parameter for the fundamental mode. The complex permittivity is estimated by inverting the measured parameters using by an algorithm based on Artificial Neural Network (ANN). The proposed sensor is extremely compact and guarantees a good accuracy for low permittivity material. The design procedure is described and experimental results are reported validating the sensor performance.