Abstract
In this paper, a comprehensive review of portable microwave sensors for monitoring moisture content (MC) is presented. MC monitoring is crucial in different industries, particularly food and farming. Microwave-based approaches for measuring the MC of the grains and mineral materials are studied. These approaches are categorized into three groups: S-parameters, dielectric constant, and impedance measurements. While these methods are interrelated, they have differences. The investigated methods use different microwave antenna sensors for MC monitoring, such as coaxial probes, horn antennas, loop antennas, microstrip patch antennas, and frequency selective surface (FSS) antenna. State-of-the-art microwave sensors were investigated thoroughly to clarify the current challenges and possible solutions of MC monitoring. A comparison between the investigated sensors was made to determine their advantages and disadvantages. According to the comparison, sensors operating above 10 GHz suffer from cross-interference. Moreover, microstrip patches can monitor a wide MC range as extensive as 60%. At the same time, the FSS sensor has the highest sensitivity with an error as low as 0.023% at X-band. Microstrip patch and FSS antennas can be printed directly on a flexible, low-loss, and lightweight material to monitor the grain MC. The flexibility, compactness, portability, ease of environment-friendly fabrication, and high sensitivity are among the criteria determining the most suitable microwave sensors for industrial and consumer MC monitoring applications.
Highlights
Monitoring the moisture content (MC) is crucial in precision farming and the food industry as an indicator of the quality of grains, fruits, and foods [1]−[13]
Measuring the dielectric constant can be done by monitoring the S-parameters or the time delay of the reflected signal from the open side of the coaxial probe inserted into the sample under test (SUT) [45]
Monitoring MC is crucial in different fields, including farming and the food industry
Summary
Monitoring the moisture content (MC) is crucial in precision farming and the food industry as an indicator of the quality of grains, fruits, and foods [1]−[13]. Since the dielectric constant of the water at some frequency bands changes with the temperature variations, sensors with no/minor cross-interference sensitivity are more desirable. State-of-the-art microwave sensors for MC measurement based on measuring S-parameters, dielectric constant, and impedance are investigated and compared with each other
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