In this paper, a novel class of sensors composed of coupled-line sections dedicated for complex permittivity measurements of dielectric samples have been proposed. The end-shorted and open-ended coupled-line sections have been used as sensing elements for nonresonant and nondestructive measurements allowing for characterization of dielectric samples. It has been shown that by utilization of an iterative procedure, the complex permittivity of a dielectric material under test can be determined based on the measured complex odd-mode effective permittivity of the system. Since the complex odd-mode effective permittivity is mainly related to the mutual capacitance between coupled strips, high sensitivity on the covering sample has been obtained and the measured sample can be as narrow as the spacing between the coupled strips. The proposed sensors have been theoretically investigated and experimentally validated. The obtained results have been compared and the sensors' advantages and limitations have been discussed. The obtained measurement results have proved the correctness and usefulness of the proposed approach.

In this article, a novel microwave microfluidic coupled-line sensor allowing for monitoring of proportional quantities and determination of complex permittivity of liquid dielectric samples through simple analytical formulas is proposed. The application of broadside coupling configuration enabled the sample to be placed in-between coupled strips, thus a high sensitivity can be obtained as the electric field for differential excitation is confined within the sample's volume. Moreover, the use of 3-D printing technology allowed for the realization of an insert with the microfluidic channel and dedicated liquid inlet/outlet. The sensitivity analysis of such a sensor is provided along with the system calibration technique, which is straightforward because of a fixed and well-defined geometry of the sensor-sample setup. An exemplary, general-purpose sensor along with a cal-kit was realized in a stripline structure and manufactured using a combination of laminate and PolyJet 3-D printing technologies. The broadband measurement results are provided for a water-ethanol solution with variable solvent content up to 12 GHz and for various alcohols featuring frequency-dependent complex permittivity up to 5 GHz. The sensor's performance is assessed and discussed.

A novel robust approach for permittivity measurements of liquid materials in a wide frequency range is proposed. A high-sensitivity microwave-fluidic sensor is developed enabling direct determination of liquid samples' permittivity from the measured sensor's propagation constant. Importantly, neither sensor calibration using liquid standards nor sensor's geometry (except length) information is required, which is very advantageous for industrial applications. This is achieved through the sensor's geometry, which is a two-wire transmission line (TWL) encapsulated in a 3-D printed container, and thus, the liquid-under-test fully encloses the propagating electromagnetic waves. The measurement setup comprises the sensor cascaded between two microstrip-to-TWL transitions for which a simplified de-embedding procedure is proposed using only a THRU standard. For demonstration, a setup was developed with transitions operating within 1-10 GHz and tested up to 18 GHz by measurement of a set of alcohols to produce useful data within the transition bandwidth, which can be extended at the cost of increased uncertainty by postprocessing. Finally, the measurement error study has shown that a relatively low uncertainty and error with respect to the reference is obtained, verifying the usability of the developed measurement technique.

A novel method for the measurement of complex permittivity of liquid dielectric materials in a wide frequency range is proposed. The technique utilizes an open-ended two-wire transmission line sensor that is submerged in a liquid-under-test to determine its complex permittivity from the measured differential reflection coefficient. The proposed approach has been theoretically analyzed to derive the appropriate sensor’s model and the required calibration procedure as well as experimentally validated by measurements of three liquids having frequency-dependent complex permittivity. Moreover, measurement accuracy analysis has been provided, including the method‘s uncertainty estimation and error sources’ discussion. Finally, the measured permittivity of each liquid has been compared against tabularized data. Good agreement has been obtained proving the validity and applicability of the proposed method.