A microwave microfluidic sensor for dielectric characterization of liquids in real time is presented in this paper. The sensor is implemented in microstrip technology and consists of a symmetric splitter/combiner configuration loaded with a pair of identical split ring resonators (SRRs) and microfluidic channels placed on top of them (gap region). The sensor works in differential mode and sensing is based on frequency splitting. Thus, if the structure is unloaded or if it is symmetrically loaded with regard to the axial plane, only one transmission zero (notch) in the frequency response appears. However, if the axial symmetry is disrupted (e.g., by the presence of different liquids in the channels), two transmission zeros arise, and the difference in magnitude (notch depth) and frequency between such transmission zeros is indicative of the difference in the dielectric properties (complex dielectric constant). A circuit schematic, including transmission line sections to describe the distributed components, lumped elements to account for the SRRs and their coupling to the lines and lumped elements to model the liquid properties, is presented and validated. After proper calibration, the functionality of the proposed sensor is demonstrated by measuring the complex permittivity in solutions of deionized water and ethanol as a function of the ethanol content.

This paper focuses on the analysis of splitter/ combiner microstrip sections where each branch is loaded with a complementary split ring resonator (CSRR). The distance between CSRRs is high, and hence, their coupling can be neglected. If the structure exhibits perfect symmetry with regard to the axial plane, a single transmission zero (notch) at the fundamental resonance of the CSRR, arises. Conversely, two notches (i.e., frequency splitting) appear if symmetry is disrupted, and their positions are determined not only by the characteristics of the CSRRs but also by the length of the splitter/combiner sections. A model that includes lumped elements (accounting for the CSRR-loaded line sections) and distributed components (corresponding to the transmission lines) is proposed and used to infer the position of the transmission zeros. Frequency splitting is useful for the implementation of differential sensors and comparators based on symmetry disruption. Using the model, the length of the splitter/combiner sections necessary to optimize the sensitivity of the structures as sensing elements is determined. Parameter extraction and comparison with electromagnetic simulations and measurements in several symmetric and asymmetric structures is used to validate the model. Finally, a prototype device sensor/comparator based on the proposed CSRR-loaded splitter/combiner microstrip sections is presented.

In this paper, an analytical method to estimate the complex dielectric constant of liquids is presented. The method is based on the measurement of the transmission coefficient in an embedded microstrip line loaded with a complementary split ring resonator (CSRR), which is etched in the ground plane. From this response, the dielectric constant and loss tangent of the liquid under test (LUT) can be extracted, provided that the CSRR is surrounded by such LUT, and the liquid level extends beyond the region where the electromagnetic fields generated by the CSRR are present. For that purpose, a liquid container acting as a pool is added to the structure. The main advantage of this method, which is validated from the measurement of the complex dielectric constant of olive and castor oil, is that reference samples for calibration are not required.

Differential techniques are widely used in communication and sensor systems, as these techniques have been shown to improve the performance. This paper shows how differential sensing of permittivity can be conducted in a simple way. For that purpose, a microstrip line loaded with a pair of stepped-impedance resonators is used in two different resonator connections: parallel and cascade. Each resonator is individually perturbed dielectrically so that: 1) when the two individual permittivities are identical, the structure exhibits a single resonance frequency and 2) when the permittivities are different, resonance frequency splitting occurs, giving rise to two resonances (all these resonances are seen in the form of transmission zeroes). The two sensing approaches are successfully validated through electromagnetic simulations and experiments. By virtue of a differential measurement, robustness against changing ambient factors that may produce sensor miscalibration is expected.

This letter is focused on the modeling, analysis, and applications of microstrip lines loaded with pairs of electrically coupled complementary split-ring resonators (CSRRs). Typically, these epsilon-negative (ENG) metamaterial transmission lines are implemented by loading the line with a single CSRR (etched beneath the conductor strip) in the unit cell. This provides a stopband in the vicinity of the CSRR resonance. However, by loading the line with a pair of CSRRs per unit cell, it is possible to either implement a dual-band ENG transmission line (useful, for instance, as a dual-band notch filter), provided the CSRRs are tuned at different frequencies, or to design microwave sensors and comparators based on symmetry disruption (in this case by using identical CSRRs and by truncating symmetry by different means, e.g., asymmetric dielectric loading). The design of these CSRR-based structures requires an accurate circuit model able to describe the line, the resonators, and the different coupling mechanisms (i.e., line-to-resonator and inter-resonator coupling). Thus, a lumped element equivalent circuit is proposed and analyzed in detail. The model is validated by comparison to electromagnetic simulations and measurements. A proof-of-concept of a differential sensor for dielectric characterization is proposed. Finally, the similarities of these structures with coplanar waveguide transmission lines loaded with pairs of SRRs are pointed out.

This paper presents a one-port reflective-mode phase-variation microwave sensor consisting of a coplanar waveguide (CPW) transmission line terminated with a grounded open complementary split ring resonator (OCSRR). The sensor is useful for measuring the dielectric constant of the so-called material under test (MUT), which should be placed in contact with the OCSRR, the sensitive element. The output variable is the phase of the reflection coefficient. Design guidelines for the implementation of highly sensitive sensors are derived in the paper, and validated through simulation and experiment. As compared to other reflective mode phase-variation sensors based on open-ended sensing lines, the designed and fabricated devices exhibit a very small sensitive region by virtue of the use of an electrically small resonant element, the OCSRR. The relevant figure of merit, defined as the ratio between the maximum sensitivity and the size of the sensing area (expressed in terms of the squared wavelength), is as high as FoM = 5643°/λ ² in one of the reported prototypes. Moreover, the paper analyzes the effects of losses. From this study, it is concluded that MUT losses do not significantly affect the output variable, provided losses are small. It is also demonstrated that the sensor is useful to estimate the loss tangent of the considered MUT samples.