  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 (OLRs) coupled to
transmission lines (TLs). Although the strongly resonant properties of the OLR
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 OLR and the TL. This work introduces a novel
solution to this problem that employs negative-refractiveindex TL (NRI-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 (SFA) 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.
