Strongly Enhanced Sensitivity in Planar Microwave Sensors Based on Metamaterial Coupling

Limited sensitivity and sensing range are arguably the greatest challenges in\nmicrowave sensor design. Recent attempts to improve these properties have\nrelied on metamaterial- (MTM-) inspired open-loop resonators (OLRs) coupled to\ntransmission lines (TLs). Although the strongly resonant properties of the OLR\nsensitively reflect small changes in the environment through a shift in its\nresonance frequency, the resulting sensitivities remain ultimately limited by\nthe level of coupling between the OLR and the TL. This work introduces a novel\nsolution to this problem that employs negative-refractiveindex TL (NRI-TL) MTMs\nto substantially improve this coupling so as to fully exploit its resonant\nproperties. A MTM-infused planar microwave sensor is designed for operation at\n2.5 GHz, and is shown to exhibit a significant improvement in sensitivity and\nlinearity. A rigorous signal-flow analysis (SFA) of the sensor is proposed and\nshown to provide a fully analytical description of all salient features of both\nthe conventional and MTM-infused sensors. Full-wave simulations confirm the\nanalytical predictions, and all data demonstrate excellent agreement with\nmeasurements of a fabricated prototype. The proposed device is shown to be\nespecially useful in the characterization of commonly-available\nhigh-permittivity liquids as well as in sensitively distinguishing\nconcentrations of ethanol/methanol in water.\n