Strongly Enhanced Sensitivity in Planar Microwave Sensors Based on Metamaterial Coupling

• Published in: IEEE transactions on microwave theory and techniques Vol. 66; no. 4; pp. 1843 - 1855
• Main Authors: Abdolrazzaghi, Mohammad, Daneshmand, Mojgan, Iyer, Ashwin K.
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.04.2018; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Artificial materials; Couplers; Coupling; Couplings; Ethanol; Linearity; Metamaterials; metamaterials (MTMs); Microwave sensors; Microwave theory and techniques; MTM transmission lines (TLs); Permittivity; permittivity measurement; Properties (attributes); Resonant frequency; Resonators; Sensitivity; Sensitivity analysis; Sensitivity enhancement; Sensors; signal-flow analysis; Transmission lines
• ISSN: 0018-9480; 1557-9670
• DOI: 10.1109/TMTT.2018.2791942

Limited sensitivity and sensing range are arguably the greatest challenges in microwave sensor design. Recent attempts to improve these properties have relied on metamaterial (MTM)-inspired open-loop resonators coupled to transmission lines (TLs). Although the strongly resonant properties of the resonator sensitively reflect small changes in the environment through a shift in its resonance frequency, the resulting sensitivities remain ultimately limited by the level of coupling between the resonator and the TL. This paper introduces a novel solution to this problem that employs negative-refractive-index TL MTMs to substantially improve this coupling so as to fully exploit its resonant properties. A MTM-infused planar microwave sensor is designed for operation at 2.5 GHz, and is shown to exhibit a significant improvement in sensitivity and linearity. A rigorous signal-flow analysis of the sensor is proposed and shown to provide a fully analytical description of all salient features of both the conventional and MTM-infused sensors. Full-wave simulations confirm the analytical predictions, and all data demonstrate excellent agreement with measurements of a fabricated prototype. The proposed device is shown to be especially useful in the characterization of commonly available high-permittivity liquids as well as in sensitively distinguishing concentrations of ethanol/methanol in water.