Stub-Loaded Patch Antenna for Development of High Sensitivity Crack Monitoring Sensor

This article proposes the use of a wireless sensor developed with a microstrip patch antenna in conjunction with an open-circuited stub for remote, real-time monitoring of crack width expansion. Technically, the open-circuited stub is exploited as a sensing structure with excellent sensitivity, and the crack growth is able to be mechanically mapped to the stub length extension via an incorporation of a mirrored stub structure placed right on top of the open-circuited stub. Thanks to a very strong relationship between the stub input admittance and its electrical length, the operating frequency of the patch is able to be significantly downshifted as the stub is mechanically lengthened (i.e., the crack grows). With the proposed wireless sensing scheme, the crack width expansion can be determined by identifying the dominant frequency component of the sensing signal received at remote data stations in a real-time manner. The sensor operating at 4 GHz was developed for experimental verification and as a demonstration of effectiveness. The experimental results show that the wireless sensor is able to identify the crack expansion of up to 2 mm with a resonant frequency downshift of 110 MHz. Furthermore, the maximum expansion and the finest increment can be simply specified with the sensor operating frequency regime. Moreover, a stable wireless link can be sustained as the patch radiation patterns remain unaffected while crack grows, and the sensor can be reused since the proposed monitoring does not result in any structure destruction or deformation.