RF evanescent-mode cavity resonator for passive wireless sensor applications

This paper presents a radio-frequency (RF) evanescent-mode cavity resonator for passive wireless sensor applications. The evanescent-mode resonator is composed of a cavity with a center post. The resonant frequency of the resonator is determined by the dimension of the cavity, the gap between top membrane electrode of the cavity, and the center post. In a wireless airflow sensor application proposed in this paper, the top electrode of the cavity resonator is deformed when airflow is applied. This results in a resonant frequency shift. A coplanar waveguide (CPW) coupling method has also been proposed for the sensor structures. The proposed coupling method has demonstrated low transition loss with a relatively simplified signal feeding structure, resulting in easy integration of sensing elements and RF antenna. The fabrication of the proposed flow sensor is realized by a simple molding technology. Measurements of airflow velocity using the proposed flow-sensor have been demonstrated with sensitivity of 0.37 GHz/(m/s). Passive wireless interrogation has also been achieved by integrating the sensor with a pair of ultra-wide-bandwidth (UWB) antennas.