Dielectric Measurement using Planar and Curved Microstrip Line

In this paper, a microwave transmission line sensor is investigated and used for bulk liquid permittivity measurement. A microstrip type transmission line is designed and analysed for a frequency range of 150 kHz to 8 GHz. The transmission coefficient magnitude of the microstrip line covered with different liquid samples is calculated numerically and compared with the measurement results. The calculated transmission coefficient magnitudes show good agreement with the normalized measurement results for the sample with a dielectric constant of less than 30 and a frequency range up to 6 GHz. A specific focus of the work is to study the transmission response of the microstrip line sensor when it is implemented on a curved substrate instead of the traditional planar substrate. A semi-flexible microstrip line is designed using a semi-rigid substrate for different curvature and its transmission coefficient is calculated numerically. The curved microstrip line sensor shows good agreement between the calculated and the measured results with minimum effect on the curvature. Furthermore, dielectric constant empirical models are formed and tested for unknown liquid samples. The results show a good match between the microstrip line sensor and the commercial dielectric probe for the determination of dielectric constant at frequency range less than 0.5 GHz and 4 to 8 GHz. The study demonstrated that the empirical models applied for the dielectric constant determination can be developed for low dielectric constant liquid despite of the sensor structure is curved or planar.