A Novel Microwave Sensor Based on Modified Rat-Race Coupler for Extracting Real Permittivity and Concentration of Binary Aqueous Solution

A novel microwave sensor based on modified rat-race coupler for extracting complex permittivity of binary aqueous solution is proposed in this manuscript. The proposed microwave sensing system consists of the modified rat-race coupler, Gain/phase RF detector, and voltage-controlled oscillator (VCO). The modified rat-race coupler is constituted by three cascaded rat-race couplers, each rat-race coupler is developed from a traditional rat-race coupler, and a 50 Ω characteristic impedance transmission line port of traditional coupler is replaced by a muti-stepped impedance transmission line (MSITL), which is used as sensing area. For a rat-race coupler, its sensitivity is determined by the added MSITL, thus the change rate between phase of reflection coefficient and concentration/real permittivity of sample for MSITL should realize the maximum value. Mathematical analysis provides a detailed explanation for what values of characteristic impedance and electrical length of each discontinuous segment of MSITL should be taken to achieve the maximum sensitivity. The relationship between concentration/real permittivity and phase of reflection coefficient can be obtained by the passive sensor, i.e., modified rat-race coupler, while the expensive vector analyzer network (VNA) is essential in measurement. In measurement, the VCO generates the RF signal source at 2.5 GHz for the microwave sensing system. And, the gain/phase detector chip AD8302 is adopted to acquire the relation between permittivity and output DC voltage, thus the output DC voltage can be used to predict the unknown concentration/real permittivity of sample and it can get rid of the use of VNA. For the modified rat-race coupler cascaded by three rat-race couplers, the magnitude of b ₁ / b ′′ ₄ ( b ₁ and b ′′ ₄ are the output powers for the ports of modified rat-race coupler) is larger than the one of single rat-race coupler, and the larger magnitude of b ₁ / b ′′ ₄ denotes the larger output DC voltage for V_mag . In measurement, the modified rat-race coupler has the maximum sensitivity of about 1.37°/ε′ _r . And the average sensitivity for the microwave sensing system with third-order rat-race coupler combined is about 2.14 mV /ε′ _r , which is 1.93 ~ 3.28 times higher than single- and second-order rat-race coupler-based ones. All in all, the proposed microwave sensing system is an economical electronic device in the field of microfluidic detection.