Sensors are key components in structural health monitoring. Although significant improvements have been made on monitoring technologies by using many advanced smart sensors, the need of cables and power supplies still causes much inconvenience in the practical implementation of these technologies. In this paper, we propose a novel rectangular patch antenna to monitor the structural health in a wireless and passive manner so that the drawback of traditional monitoring methods can be fully overcome. The antenna is used as a sensor and its resonant frequency will change linearly in terms of its size changes in both horizontal and vertical directions. The antenna will transmit an electromagnetic wave with the resonant frequency information to a reader by a long distance and activate the chip in the reader. According to the received information about the resonant frequency, the change of the targeted structure's strain can be measured in real time and its health state can then be evaluated. Simulations and experiments show that the designed antenna is an excellent strain sensor and can be applied to the health detection for many types of structure.

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When sensors based on a single patch antenna are used to monitor crack width, the issues of incomplete strain transfer ratio, insufficient bonding strength, and randomness of crack propagation will inevitably compromise sensor sensitivity and make the calibration uncontrollable. To circumvent these difficulties, this study presents a novel crack-measuring sensor based on a rectangular patch antenna fed by a pair of microstrip lines, which form a parallel plate capacitor as a crack-sensing unit. The sensing mechanism was analyzed using the transmission-line model. Then two crack sensor prototypes were designed and the dimensions were optimized. A crack opening simulator was customized to test the sensors by using different width resolutions. The experiment results validated that the resonant frequency shifts are linearly proportional to the applied crack width; moreover, the crack sensor proved capable of detecting crack widths as small as one-hundredth of a millimeter in an object.

Monolithic patch antennas for deformation measurements are designed to be stressed. To avoid the issues of incomplete strain transfer ratio and insufficient bonding strength of stressed antennas, this paper presents a passive wireless crack sensor based on an unstressed patch antenna. The rectangular radiation patch of the proposed sensor is partially covered by a radiation sub-patch, and the overlapped length between them will induce the resonate frequency shift representing the crack width. First, the cavity model theory is adopted to show how the resonant frequencies of the crack sensor are related to the overlapped length between the patch antenna and the sub-patch. This phenomenon is further verified by numerical simulation using the Ansoft high-frequency structure simulator (HFSS), and results show a sensitivity of 120.24 MHz/mm on average within an effective measuring range of 1.5 mm. One prototype of proposed sensor was fabricated. The experiments validated that the resonant frequency shifts are linearly proportional to the applied crack width, and the resolution is suitable for crack width measuring.

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