A Compact Differential Microwave Fluid Sensor for Permittivity Measurement of Ethanol–Water Solution

In this article, a novel approach is proposed to address sensors’ significant challenges by introducing a portable planar microstrip sensor based on a dual-bandpass filter (BPF). This sensor is designed specifically for permittivity measurement of water-ethanol solution and offers high sensitivity, low cost, and reusability while minimizing the environmental unwanted effects. The proposed sensor consists of a microstrip line structure with two crescent arms as host, and an innovative sensing area is coupled to it. The sensing mechanism relies on the manipulation of the transmission resonance frequency. Notably, this configuration exhibits two transmission poles (TPs) in the transmission coefficients at specific frequencies, namely, 1.6 and 4.48 GHz. The second TP is particularly sensitive to changes in ethanol concentration (EC) and ambient variations too. On the other hand, the first TP remains unaffected by variations in the sample under test (SUT). To validate the effectiveness of the proposed sensor, a prototype was fabricated on RO4003C dielectric substrate, and subjected to rigorous testing using various ECs in deionized water. The prototype sensor boasts high sensitivity and compact size of $8\times 7.2\times 0.508$ mm, which is advantageous for practical applications where space constraints are a concern.