Highly sensitive DS-CSRR based microwave sensor for permittivity measurement of liquids

Abstract

In this work, a low-cost Double Split Complementary Split Ring Resonator (DS-CSRR) microwave sensor has been designed and fabricated on an FR4 substrate for permittivity characterization of liquids. This modified CSRR structure is proposed to make use of the benefits of the dielectric resonator technique compared to other types of microwave methods. The resonator structure comprises two slits in each of the rings normal to each other while maintaining one slit in each ring along the feed line for maximum excitation of the resonator. The resonator with substrate dimensions of 20mm×30mm acts like an LCR circuit and operates at 2.501 GHz of the Industrial Scientific and Medical (ISM) band. Eleven different liquid samples (each with 150 μL of volume) covering a wide permittivity range of 1-111 have been used to measure the transmission coefficient (S_21) using Vector Network Analyzer (VNA). In comparison with many recently reported works, the sensitivity of the sensor is found to be higher with average and normalized sensitivity of 35.07 MHz/unit permittivity and 1.4 % respectively. Fit equations are developed for both real and imaginary parts of permittivity, and using the fit equations the permittivity of three unknown samples are determined with less than 3.7 % error. The sensor adopts a unique orientation of the slits to enhance the E-field intensity for better interaction of the samples with the sensor. A compact form factor, low production cost, future integration possibilities, and high sensitivity are the distinct highlights of the sensor. The sensor promises to be a potential candidate for situations that demands low-cost and highly accurate measurements and may be a good alternative to commercial sensors for dielectric characterization, concentration analysis, and impurity measurement of liquids.