A Novel Substrate Integrated Waveguide Shorted Coaxial Resonator for Characterizing Complex Permittivity of Liquids and Solid Content of Water-Based Ferrofluid

A novel substrate integrated waveguide (SIW) shorted coaxial resonator (SCR), embedded with a dual-channel microfluidic, is proposed to implement a microwave liquid sensor. The novel SIW SCR not only demonstrates the highly concentrated E-field by innovatively inserting an annular parallel plate capacitance between the outer conductors of the SIW SCR, but also represents the uniformly distributed strong H-field intensity distribution between the inner and outer conductors of the SIW SCR, which is similar to those of the traditional coaxial line. Then, the dielectric and magnetic sensing channels in the dual-channel microfluidic are skillfully designed to be placed in the strong E- and H-field regions of the SIW SCR, which are used to characterize the complex permittivity of liquids and the solid content of water-based ferrofluid (WFF), respectively. In addition, the equivalent lumped circuit model of the SIW SCR is successfully established to analyze its sensing mechanism, and the values of the equivalent lumped elements are extracted. Moreover, a LabVIEW-based real-time test system is combined with an averaging data processing method to ensure the reliability of the measured results, and the maximum measurement errors of the complex permittivity and solid content of WFF are 3.83% and 2.26%, respectively. The SIW SCR is an excellent candidate for accurate characterize the liquid and provides technical support for the nondestructive testing of liquid materials.