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Many existing sensing technologies for application to the monitoring of civil structures have a serious deficiency in that they require some type of wired physical connection to the outside world. This causes significant problems in the installation and long-term use of these sensors. This paper describes a new type of passive wireless sensor that is based on resonant RF cavities, where the resonant frequency is modulated by a measurand. In the case of a strain sensor, the electrical length of the cavity directly modulates its resonant frequency. A probe inside the cavity couples RF signals from the cavity to an externally attached antenna. The sensor can then be interrogated remotely using microwave pulse-echo techniques. Such a system has the advantage of requiring no permanent physical connection between the sensor and the data acquisition system. In this type of sensor, the RF interrogation signal is transmitted to the sensor and then reradiated back to the interrogator from the sensor resulting in a signal strength that decreases with the forth power of distance. This places an upper limit on the distance over which the sensor can be interrogated. Theoretical estimates show that these sensors can be interrogated with sufficient SNR at distances exceeding 10 m for radiated powers of less than 1 mW. We present results for a strain sensor and a displacement sensor that can be interrogated at a distance of 8 m with a strain resolution of less than 10 ppm and displacement resolution of 0.01 mm, respectively.