Multiband cross-fractal antenna integrated with SRR metamaterials for high-sensitivity liquid characterization and optimal frequency band identification

Abstract This study presents a novel compact planar antenna based on cross-fractal geometry integrated with square split-ring resonator (SRR) metamaterials for advanced multiband liquid characterization and optimal frequency band detection. The proposed sensor synergistically combines the space-filling properties of fractal structures with the electromagnetic field enhancement capabilities of metamaterials to achieve exceptional sensitivity across a broad 1−10 GHz frequency range while maintaining an ultra-compact (10 × 10) mm 2 footprint, which corresponds to wavelength dimensions the order of 0.013 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>λ</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> </mml:msub> </mml:math> × 0.013 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>λ</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> </mml:msub> </mml:math> (where <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>λ</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> </mml:msub> </mml:math> is the free-space wavelength calculated at the lowest operating frequency), and demonstrates a high gain of 4.28 dBi. The SRR metamaterial integration significantly enhances frequency selectivity and field confinement, resulting in an impressive S 11 variation of −23 dB and a remarkable sensitivity of 300 MHz when tested with deionized water-representing a 50-fold improvement over existing sensors. A key innovation lies in the systematic identification of the optimal frequency band through comprehensive multiband analysis, where experimental validation using pure water, pure ethanol, and water-ethanol mixtures reveals that the 5 GHz band exhibits the highest sensitivity to dielectric property variations. The sensor requires only ∼100 mm 3 of test liquid, enabling point-of-care applications with limited sample availability, while rigorous evaluation of sample holder configurations ensures minimal measurement perturbations and optimal accuracy. Comparative analysis demonstrates superior performance over existing fractal and metamaterial-based biosensors in sensitivity (300 MHz versus 0.4–0.7 MHz) with a bandwidth fraction of 54.54%, sample volume requirement (100 mm 3 versus 300–5000 mm 3 ), operational bandwidth (multiband versus narrow sub-bands), and gain (4.28 dBi versus 2.28–3.15 dB). The standard printed circuit board (PCB) fabrication process ensures cost-effectiveness and scalability, while vector network analyzer calibration and excellent agreement between simulated and experimental results validate the design’s reliability. This work establishes a new benchmark for compact, high-sensitivity microwave sensors and provides valuable insights for frequency band selection optimization, with significant implications for next-generation biosensors in medical diagnostics, chemical analysis, and environmental monitoring applications.