RADIO PHYSICS AND RADIO ASTRONOMY

Subject and Purpose. In this paper, the use of a planar split-ring resonator for contactless identification of liquids inside sealed thin-walled plastic containers is demonstrated numerically and experimentally. The motivation behind this study is to determine the split-ring resonator type most suitable for distinguishing among liquids.

Methods and Methodology. A planar split-ring resonator serves as a high-frequency sensor operating at 2.3 GHz. Different liquids are identified by comparing the transmission spectra of a planar feeding strip line that is loaded with the resonator. Plastic containers filled with various liquids are positioned at different distances from the resonator. The changes in the resonator spectrum are evaluated using its two key parameters: the resonant frequency and the inverse Q-factor. Additionally, the developed spectral-analysis software reduces errors in the determination of Q-factor.

Results. The ability of the planar split-ring resonator to distinguish among liquids has been demonstrated for three liquids with different compositions. In particular, mineral water, ethyl alcohol, and refined gasoline were identified. The differences among the liquids are tracked in the changes of inverse Q-factor and resonant peak frequency of the planar split-ring resonator. In this regard, the dependences of the inverse Q-factor and the resonant peak frequency on the distance between the liquid container and the planar split-ring resonator plane were analyzed. Furthermore, it has been shown that with the proposed planar split-ring resonator as a sensor, parameters of the liquid containers affect liquid-identification accuracy much less than they do when a photonic crystal with a defect is employed.

Conclusions. A real advantage of the proposed planar split-ring resonator is its low sensitivity to the container parameters. This was experimentally verified while moving a 0.5-liter (16.9 FL oz) liquid-filled polyethylene terephthalate bottle along a straight line 2 mm away from the resonator plane.

Keywords: contactless liquid identification, split-ring resonator, microwave frequencies, inverse Q-factor, permittivity

Manuscript submitted 23.04.2026

Radio phys. radio astron. 2026, 31(3): 199-206

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