Microwave sensors for dielectric characterization of materials: a review

Abstract Due to excellent performance of microwave sensors, these are increasingly being deployed in wide variety of applications, such as in food quality assessment, medical diagnosis, agriculture, environment monitoring, and many more. The microwave sensors offer several benefits over other traditional sensing techniques, such as low cost, simple fabrication, high sensitivity, non-invasive, easy operations, compact design, etc. Microwave sensors can be used to analyse both physical and chemical properties of given materials, such as dielectric properties, concentration levels, imperfections, defects, non-uniformities, deformities, or impurities. The paper provides a detailed analysis of microwave sensors in material characterisation by covering their working principles, design strategies, sensor types, sample loading techniques, and their applications. In this review, the classification of microwave sensors is systematically done as per their functional purposes (e.g., fault detection, concentration measurement, and impurity detection), application domains (healthcare, agriculture, environment, and industry), operating mechanisms (resonance-based, frequency/phase variation, and frequency splitting methods), and sensor types (planar, waveguide, and free-space setups). Further, this review also discusses the current challenges faced by microwave sensors, such as the requirement for calibration, a vector network analyzer (VNA), variation due to temperature fluctuations, and external electromagnetic interference. Lastly, the future outlook for producing market-ready sensors, fabrication techniques improvement, and integration of AI with advanced algorithms to improve sensor performance are also discussed.