Coaxial Microwave Sensor Using Defected Conductor Structure for Liquid Complex Permittivity Measurement

This paper proposes and implements a coaxial microwave sensor (CMS) based on a defected conductor structure. By etching a meander structure reshaped by a regular octagon (MSRRO) on the outer conductor of the coaxial structure, it achieves high-sensitivity characterization of the dielectric properties of liquid media across a broad measurement range. The core principle of this design is to reconstruct the electromagnetic field distribution around the axis by introducing periodic or non-periodic defected patterns, thereby enhancing the circumferential induction and field confinement capabilities. Moreover, this research establishes a complex permittivity inversion model based on relative frequency shift and frequency concentration ratio. Simulation results demonstrate that the relative error in the real part of permittivity is less than 2.17% with an average error of 0.92%, while the absolute error in loss tangent remains below 0.01. Experimental measurements reveal that the working frequency of the proposed CMS is 886 MHz, exhibiting significant relative frequency shifts of up to 55.19% over a relative permittivity range of 1.015 to 10.09, and 81.26% over an extended range of 1.015 to 76.175. The corresponding average sensitivities for these ranges are 9.51% and 8.06%, respectively.