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.