A patch antenna-based strain sensor for structural health monitoring

A passive strain sensor was investigated using patch antennas. For early damage detection in structures due to external loads, reliable strain information is necessary. A noninvasive method of measuring strain using a patch antenna was investigated to overcome the limitations of existing strain sensing technologies. The metal patch antenna was made on a thin sheet of low-loss polymer with a ground plane on the opposite side. When fed with RF signals, the patch antenna radiates at its resonant frequency. The resonant frequency of the patch antenna varies with its dimensions. Strain-induced change in the dimensions results in a shift in the resonant frequency. A single-frequency antenna, designed and simulated using the Sonnet simulation tool, has a resonant frequency corresponding to its length, so the antenna is sensitive only to the length-direction strain yet insensitive to the width-direction strain. Effect of strain on frequency shift and its sensitivity to strain were calculated. The antenna was fabricated using conventional micromachining techniques. Effects of strain on resonant frequency were verified experimentally and in good agreement with simulated results.