Planar Sensing Platform Based on Split Ring Resonators and Microstrip Yagi-Uda Antennas

Exciting microwave split ring resonators (SRRs) with an adjacent transmission line (TL) can put additional loading effect on the resonant characteristics and compromise the selectivity and resolution of SRR sensors. This work presents an alternative approach that reduces the near-field coupling of SRRs (to TL probes), by positioning them in Fresnel zone of two printed Yagi-Uda antennas, leading to a reduction in the loading effects of external coupling/probing circuitry. Therefore, the need for TLs is eliminated and the impact of external circuitry on the sensing performance is reduced. To achieve a small footprint, the overall sensing platform, including the SRRs, antennas, and director elements, was designed at ${K} _{u}$ band and integrated on a single substrate of size $151\times 60\times0.7$ mm3. Experimental results indicated that for $6\times6$ mm solid samples, the sensitivity of the resonant frequency to permittivity variations was 69.19 MHz/ ${\varepsilon }^{\prime }$ . For liquid samples (0%–95% v/v aqueous-ethanol mixtures), the highest monitored sensitivity of the resonant amplitude was $\sim $ 1.6 dB/% of change in the ethanol concentration. These measurements demonstrated the dependency of the sensor’s $\vert {S}_{{21}}\vert $ (dB) parameters on the dielectric properties of the samples. The proposed method proves advantageous for implementing a low profile, planar sensing platform, with an excitation mechanism that eliminates TL coupling (to resonators).