Amplitude Variation Microwave Sensors for High-Sensitivity Detection of Solid and Microfluidic Dielectrics

This article presents the design of high-sensitivity and low-cost microwave sensors using a transmission line loaded with a composite resonator formed by a shunt connection of a stepped impedance resonator (SIR) and a grounded inductive strip. The proposed sensor produces a transmission zero together with a transmission pole, where applying any dielectric sample to the sensor modifies both the zero and pole frequencies. Thus, the sensor can operate based on the frequency shift principle as well. However, by a proper design of the transmission pole and zero, the sensor shows large variations in the
$S_{21}$
amplitude that is measurable at a single frequency. This offers a simplified and low-cost measurement even using a fixed frequency signal source and a power detector. The operation principle and design procedure of the sensor are explained using analytical circuit models. The presented sensing approach can be applied to detect both low-loss and lossy dielectric samples. As a verification, a sensor prototype was designed for the dielectric constant detection of low-loss microwave substrates and a microfluidic sensor for water content detection in the water–ethanol solutions.