A quarter-wavelength folded patch antenna is adopted as the passive wireless strain sensor for structural health monitoring (SHM) of bridges. It can be used for continuous surveillance and damage detection. According to theoretical formulations, strain simulation and experiments, it is found that a good linearity relationship can be achieved between normalized resonance frequency shift and the strain both in longitudinal and transverse directions. And the sensing sensitivity in longitudinal strain is better than that in transverse strain. Through conducting tensile experiments, we find that many factors can influence the strain sensitivity. To address this fundamental issue in antenna sensors for strain sensing, a new strain sensitivity experiment is proposed to take the influence of strain transfer ratio change under strain. The linear relationship of strain transfer ratio and deformation is obtained by sensitivity experiment. The corrected sensitivity in longitudinal and transverse strains is calculated based on the linearity. Furthermore, the Possion effect is taken into consideration to explain the opposite effects of experimental and simulated sensitivities in transverse strain.
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