High-Sensitivity Slot-Loaded Microstrip Patch Antenna for Sensing Microliter-Volume Liquid Chemicals with High Relative Permittivity and High Loss Tangent

This paper proposes a microwave sensor based on a high-sensitivity slot-loaded rectangular microstrip patch antenna (MPA) for measuring microliter-volume liquid chemicals with high relative permittivity and high loss tangent. A rectangular single-ring complementary split ring resonator (SR-CSRR) slot with a bottom-edge center split (BCS) was inserted along the upper radiating edge of the patch to enhance the relative permittivity sensitivity of the MPA. The first resonant frequency of the proposed SR-CSRR-BCS slot-loaded MPA showed the highest sensitivity compared to the resonant frequencies of the MPAs with other commonly used slots for varying the relative permittivity of the planar substrate type material under test from 1 to 10 when placed above the patch. After designing the scaled SR-CSRR-BCS slot-loaded MPA with the unloaded first resonant frequency at 2.5 GHz, a hollow acrylic cylindrical liquid container with an inner volume of approximately 18.6 μL was placed at the top-edge center of the SR-CSRR-BCS slot to achieve maximum sensitivity. A quarter-wavelength transformer was applied between the patch and the feed line of the MPA to improve the impedance mismatch that occurs when liquid chemicals with a high loss tangent are placed in the container. Water, methanol, and ethanol were carefully selected for test liquids to cover a broad range of relative permittivity and high loss tangents. The proposed SR-CSRR-BCS slot-loaded MPA was designed and fabricated on a 0.76 mm-thick RF-35 substrate, and a reference RS-loaded MPA was designed and fabricated for comparison. The shift in the first resonant frequency of the input reflection coefficient characteristic was used for the sensitivity comparison, and the container was filled with 15 μL of the liquids at 25 °C. The measured sensitivity (%) of the proposed SR-CSRR-BCS slot-loaded MPA for water was 0.45%, which was higher than other antenna-based microwave sensors in the literature.