Novel Microwave Sensors Based on Split Ring Resonators for Measuring Permittivity

Measuring the electromagnetic properties of materials has important applications in many fields. In this paper, two electrically small sensors based on the split ring resonators (SRRs) with extended long legs, i.e., two-layer and three-layer magnetic coupled SRRs, are proposed to measure the permittivity of small samples of unknown materials. By virtue of the characteristics of SRRs, the proposed resonators have several merits, such as extremely compact size (
$0.067\lambda _{0}\times 0.067\lambda _{0}$
and
$0.052\lambda _{0}\times 0.052\lambda _{0}$
for two-layer and three-layer magnetic coupled SRRs, respectively), high quality factor (
$Q$
), and stable resonance. Especially, the proposed three-layer magnetic coupled sensor with opposite splits on the SRRs is able to further improve the quality factor and have better stability compared with the two-layer coupled sensor. By different shifting resonant frequencies instead of the single ones and the polynomial fitting method, the proposed sensors can accurately calculate the unknown permittivity. Simulated and experimental results have validated the efficacy of the proposed approach and designs. With the features of compact size and lower far-field radiation, the proposed resonators can be combined with various permittivity measurement algorithms to improve the measurement accuracy in a wide range of environments beyond the specific experimental setup.