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

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 combines the space-filling properties of fractal structures with electromagnetic field enhancement capabilities of metamaterials to achieve exceptional sensitivity across a broad 1-10 GHz frequency range while maintaining a compact (10×10) mm2 footprint and high gain of 4.28 dB. The SRR metamaterial integration significantly enhances frequency selectivity and field confinement, resulting in an impressive S₁₁ 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 mm³ 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 vs. 0.4-0.7 MHz), sample volume requirement (100 mm3 vs. 300-5000 mm3), operational bandwidth (multiband vs. narrow sub-bands), and gain (4.28 dB vs. 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.