Research on the Dielectric Constant Sensor System Based on the Short-Ended Transmission Line

This paper presents a sensor system for measuring the dielectric constant of materials, with a control circuit, a broadband coupler and a sensing probe as its main components. The entire system can independently measure the dielectric constant of powdered solid materials and liquid materials without relying on any auxiliary equipment. According to the transmission line design theory, the sensing probe utilizes a short-ended transmission line based on air coplanar waveguide. By introducing a parallel resistor at the input of the transmission line, material under test (MUT) with different dielectric constant can directly influence the resonance frequency of the transmission line's reflection coefficient. The dielectric constant can be inverted through the measurement of the resonance frequency. This paper systematically analyzes the sufficient condition for the validity of the conclusion and discusses interference factors that may affect the measurement result, such as variation in resistor value and difference in material loss. Subsequently, a dedicated control circuit was designed and manufactured by integrating a control chip, an RF chip, and a detector, thus forming a low-cost, portable sensor system together with the sensing probe and coupler. Experimental tests were conducted on six different MUTs, and the results confirmed that the system has the capability to accurately characterize the dielectric constant of powdered solid and liquid material. Compared to other sensor systems, the proposed system features a simplified design and higher measurement precision, indicating its broad applicability in radio frequency identification.