This article presents a novel sensor with very high sensitivity for label-free multivariable analysis of water–clay–bitumen at high temperatures based on microwave sensing techniques. The presented sensor is a single-wire single-port helical structure forming an inductor at lower frequencies. Multivariable sensing capability is enabled due to the uniqueness of the dielectric permittivity spectrum of different materials and utilizing more than one harmonic for data analysis. The parasitic capacitance of the single-wire structure in par with its inductance forms resonance harmonics, which could happen at different frequencies. The extreme sensitivity of this sensor is due to the direct relation of the capacitance of the structure to the real part of the dielectric permittivity of the ambient material rather than the dielectric permittivity of the substrate for the planar microwave sensors. In addition to increasing the sensitivity, removing the substrate, which results in a pure metal structure for the sensor, brings the benefit of the capability of the proposed sensor to work in harsh environmental conditions such as extremely high-temperature applications not achievable by planar microstrip microwave sensors. The promising overall average absolute error of less than 4% for tested samples in determining water–clay–bitumen mixtures presents the precision and capability of the presented sensor for multivariable analysis in harsh environmental conditions.
