Electrically Small Reflective Tag for Precise Water Level Monitoring

This paper proposes a compact, low-profile, chipless RF sensor for detecting water level. The system features a Vivaldi antenna (interrogator) and a meander line resonator within a small container filled with liquid (tag). The resonant frequency of the tag shifts drastically from 0.71 GHz to 1.89 GHz when just 1 mm of the meander line begins to submerge. As more water is added to the container, the resonant peaks typically decrease due to the increasing effective permittivity inside the container. Additionally, water alters the resonant mode of the meander line resonator by loading and distorting the electric field distribution on the conductive surface with a high-permittivity medium. As a result, a fundamental mode and three higher-order resonating modes can be found as more water is added. Based on this behavior, the water's level can be determined with the resonant peak, the electric field distribution, and its Q-factor if required.