T-Junction Loaded with Interdigital Capacitor for Differential Measurement of Permittivity

The microwave sensors have been successfully used for permittivity measurement. These sensors suffer from limited sensitivity and environmental effects. This article presents a novel T-junction highly sensitive microwave sensor for permittivity measurement of low-loss solid materials. The proposed sensor operation principle is based on downshifting the transmission zero (TZ) of the outputs of T-junction with the coupling of the material under test (MUT). The sensing section consists of an interdigital capacitor (IDC) located in between the lines of the T-junction. IDC is directly connected to output arms of T-junction, so that it could disturb the outputs strongly. Any change in electric field concentration in IDC directly is transmitted to the outputs and is translated as TZ change. Design steps including T-junction and IDC effects on outputs are presented in detail. The sensor operation principle is described through an equivalent circuit model, which is validated by simulation and experimental results. Two outputs of the proposed sensor show the same electrical performances, which allow differential operation mode. Hence, cross sensitivity due to environmental factors can be tolerated by the sensor. Measurement results of the fabricated prototype show 112-MHz frequency shift per unit permittivity change and a normalized sensitivity of 3.9%, which are larger than available similar sensors. The proposed sensor is implemented on a 22.22 × 18.76 × 1.6 mm3 printed circuit board.