Exploiting the Coupling Variation of 3-D-Printed Cavity Filters for Complex Dielectric Permittivity Sensing

In this work, the design of a complex dielectric permittivity sensor based on the perturbation of the coupling between two cavity is presented. The frequency response of the coupled resonators naturally presents two resonant peaks, and the reading is obtained by comparing their frequency and their insertion loss. A glass pipe is used to hold the liquid sample to be measured. The proposed layout allows to easily obtain a differential reading using a compact topology. This lowers the effect of fabrication errors on the accuracy of the readings, making it particularly useful, for example, when employing 3-D-printed components. To demonstrate the idea, a sensor working in the X-band is designed, fabricated via additive manufacturing, assembled, and measured. The prototype is tested using the mixtures of solvents (acetone and isopropanol) at different ratios. The component results in a reading with precision within ~6.7% for $\varepsilon _{r}$ and ~13.8% for tan $\delta $ .