Highly Sensitive Miniaturized Octagonal DS-CSRR Sensor for Permittivity Measurement of Liquid Samples

This work considers the use of low-cost double split complementary split ring resonator (DS-CSRR)-based microwave sensor for the measurement of complex permittivity of liquid samples with high accuracy. The sensor has been designed to operate at 2.4 GHz of the industrial scientific and medical (ISM) band, where the optimized design has been chosen in accordance with the high shift in resonant frequency when liquids under tests (LUTs) are brought in contact with the resonator. The DS-CSRR sensor exhibits a very high normalized sensitivity of 1.52% based on the experimentation of ten common liquid samples covering a wide range (1–111) of dielectric constant values. Fit equations have been developed from the information of permittivity (real and imaginary parts) against the corresponding shift in resonant frequency and the magnitude of transmission coefficient (
$S_{21}$
). With methanol and ethanol as unknown samples, their real and imaginary parts of permittivity have been successfully determined with very high accuracy. With the requirement of only microliter volume of the sample, the proposed sensor has been found to be highly capable of detecting permittivity of unknown samples as well as differentiating samples with decimal-level differences in permittivity. This promises the sensor to be an inexpensive and robust alternative to commercial sensors.