Circular Open-Cavity Resonator for Microwave Characterization of Free-Flowing Materials

Accurate, fast, and in-line determination of the microwave properties of free-flowing materials, such as gases, liquids, and powders, remains a challenge. This work details the development of a novel microwave resonant sensor based on a circular open-cavity design using a circular waveguide feed operating in the TE01 circular waveguide mode, to address this issue. This open-cavity design allows a free-flowing material under test (MUT), to pass through the cavity enabling in-line measurement of its reflection coefficient, which can then be used to estimate its effective dielectric and magnetic properties. A forward model describing the electromagnetic (EM) behavior of the sensor is derived using modal analysis and subsequently validated using full-wave simulation. The forward model facilitates accurate inversion of the measured reflection coefficient (i.e., S-parameter, S11) data for determining the effective dielectric or magnetic properties of the MUT. To demonstrate the efficacy of the sensor, measurements of an offline (static, not flowing) powder, as well as a flowing metal powder, are presented.