Abstract
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 $= 5643^{\circ }/\lambda ^{2}$ 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.
Highlights
One-port reflective-mode phase-variation sensors are very interesting devices for material characterization, including solid and liquids [1], [2]
THE PROPOSED open complementary split ring resonator (OCSRR)-BASED REFLECTIVE-MODE PHASE-VARIATION SENSOR, WORKING PRINCIPLE, AND CIRCUIT MODEL In a recent paper [1], it was demonstrated that highimpedance quarter-wavelength or low-impedance halfwavelength open-ended sensing lines are useful for the implementation of highly sensitive reflective-mode phasevariation dielectric constant sensors
We would like to mention that other phase-variation sensors based on electro-inductive wave (EIW) [40] and composite right/left-handed (CRLH) [30] artificial lines have been reported, but in such sensors the phase information was converted to magnitude information
Summary
One-port reflective-mode phase-variation sensors are very interesting devices for material characterization, including solid and liquids [1], [2]. Most planar reflective-mode phase-variation sensors are based on open-ended transmission lines [1], [2], and are devoted to the characterization of solid or liquid samples (excellent sensitivities are reported in [1]). Based on this circuit model, a sensitivity analysis of the proposed OCSRR-based sensors is carried out, where the link to the sensors based on a half-wavelength open-ended sensing line is included. THE PROPOSED OCSRR-BASED REFLECTIVE-MODE PHASE-VARIATION SENSOR, WORKING PRINCIPLE, AND CIRCUIT MODEL In a recent paper [1], it was demonstrated that highimpedance quarter-wavelength or low-impedance halfwavelength open-ended sensing lines are useful for the implementation of highly sensitive reflective-mode phasevariation dielectric constant sensors. The generalization to the complete sensor structure, including the step-impedance CPW line sections, is very simple, as will be shown below (such step-impedance configuration has a multiplicative effect on the sensitivity, dictated by the impedance contrast of the different transmission line sections)
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