High Sensitivity Sensor Loaded with Octagonal Spiral Resonators for Retrieval of Solid Material Permittivity

In this paper, a highly sensitive octagonal spiral sensor is presented that can be applied to measure the permittivity of solid materials with high accuracy. Compared with the traditional split-ring resonator (SRR)-based configuration, the confinement of the electric field in the gap of the polygon spiral resonator can be improved by increasing the number of polygon sides, which leads to higher measurement sensitivity. By compromising between ease of processing and sensitivity improvement, an octagonal spiral structure is finally obtained. To determine the measurement sensitivity, a differential measurement scheme loaded with dual octagonal spiral sensors is adopted, and the sensor is modeled under the unloaded condition to confirm its designed operating frequency of 2.48 GHz. Then, experiments and corresponding simulations are implemented on loading with two different kinds of reference materials. The equivalent circuit model is obtained, and its results are compared with those from the numerical simulation. The sensor is fabricated on a 0.5-mm-thick Teflon substrate. Two of standard solid dielectric samples are tested using the proposed scheme, and their permittivity values are retrieved. The measured data are in agreement with the corresponding reference values available in the literature, with typical errors of less than 2%.