One of the most important characteristics of planar microwave resonator-based sensors is their sensitivity. Since their measuring parameter is resonance frequency shift, the sensitivity could be described as the shift in the resonance frequency of the microwave resonators versus variation in the dielectric constant of the material under the test. Although label-free microwave resonator-based sensors are capable of providing moderate levels of sensitivity sufficient for enabling numerous applications, detection of material variations in sub-percent or ppm (part per million) range is not easily possible with these structures. This paper investigates the reasons behind the limited sensitivity of planar microwave resonator-based sensors and suggests a measurement structure capable of measuring extremely low concentrations of water contamination in mineral oil samples. Simulation and experimental results verify the capability of the presented sensing structure in the detection of as low water concentrations as 50 ppm in mineral oil samples expressing extremely high sensitivity for the presented sensor with 30 kHz in the frequency shift of the resonator.
