Open Transverse-Slot Substrate-Integrated Waveguide Sensor for Biomass Permittivity Determination

A novel open transverse-slot substrate-integrated waveguide sensor is presented. The sensor is designed and fabricated for dielectric measurements of sawdust at 8 GHz. Different configurations of the sensor were investigated by using simulation software and relationships between the simulated magnitude and phase of the reflection coefficient and dielectric properties. A suitable configuration was fabricated with a circuit board milling machine, and the performance was investigated through measurements of the magnitude and phase of the reflection coefficient from sawdust samples. The permittivity of the material was determined by fitting the voltage stand wave ratio and phase with a normalized 2-D polynomial with known dielectric values. The permittivity determination model was tested by using independent sets of training and validation measurements on sawdust at different bulk densities and moisture contents between 6.8% and 39%. The predicted dielectric properties values showed linear relationships with moisture content. Therefore, moisture content in sawdust can be determined from measurements of either dielectric constant or loss factor at a single microwave frequency without knowledge of the bulk density.