Multiband Permittivity Measurement Using Directional Coupler Based on Groove–Ridge-Gap Waveguide

In this paper, a narrowband method to measure the real part of the relative permittivity of homogeneous non-magnetic dielectric materials within the range from 1 to 10.2 is presented. To achieve multi-band dielectric property characterization, a novel coupler based on the groove-ridge-gap waveguide (GRGW) is designed. The material under test (MUT) is filled into the coupling aperture of the coupler, and the reflection coefficient is utilized as the sensor's response. Thanks to the slow-wave characteristic of the spoof surface plasmon polaritons (SSPP) mode excited by the metallic groove-ridge structure, the permittivities of MUTs can be simultaneously characterized within multiple frequency bands. In addition, samples in various states, such as solid and liquid samples, can be characterized using the proposed method, showcasing its accuracy, robustness, and universality. Finally, all the characteristics of the theoretical analysis have been fully verified by one fabricated prototype. The test results showed that the measurement errors are always less than 4% within the three frequency bands, which proves the effectiveness of the proposed method.