Double-frequency passive deformation sensor based on two-layer patch antenna

To avoid the issues of incomplete strain transfer ratio and insufficient bonding strength of a monolithic stressed antenna, this paper presents an unstressed deformation sensor based on two-layer patch antenna for structural health monitoring. The proposed sensor is composed of a monolithic patch antenna and a stacked patch generating two fundamental resonant frequencies within a 3-to-7 GHz band. The resonant frequencies' shifts caused by the offset of the stacked patch were selected as the sensing parameters. An equivalent circuit was used to analyze the sensing method, which shows the relative displacement to be linear to the shift of resonant frequencies. This phenomenon was then checked by numerical simulation using the Ansoft High Frequency Structure Simulator 15 (HFSS15) and experiments in laboratory using both wired and wireless setups. Furthermore, the accuracy of measurement is verified to be increased by combining two resonant frequencies.