In this article we describe a type of sensor to monitor strain. The strain sensor is a passive device that can be embedded or attached to a structure and then remotely interrogated though a wireless interface. Such a system has the advantage of requiring no permanent physical connection, either electrical or optical, to an interrogation system. The sensor is a conducting coaxial electromagnetic cavity that is embedded in or bonded to the structure in which strain is to be measured. The cavity will exhibit resonance for electrical wavelengths two times the cavity length. Changes in the structure's dimensions will be reflected in changes in the dimensions of the cavity, and will result in a shift of the resonant frequency of the cavity. The sensor incorporates an antenna so that the resonant frequency of the cavity can be determined by remote interrogation. The acquired resonant frequency is then used to calculate the strain in the structure. The sensor presented in this article operates at a frequency of approximately 2.45 GHz, and exhibits a shift in resonance of 2.45 kHz per microstrain ((mu)(epsilon)). We have demonstrated a strain sensitivity of less than 1 (mu)(epsilon) with a bandwidth of 25 Hz. This class of embeddable sensor is expected to have the greatest application in monitoring the health of, and assessing damage in, civil structures.
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