a wireless sensor for indicating a physical state within an environment includes a unitary housing defining a cavity . a structure located within the cavity of the housing has elements providing capacitance , the elements being arranged such that the distance and thereby the capacitance of the structure changes when a physical state of the environment changes . the structure has a resonant frequency based at least in part on the capacitance of the structure when in the presence of a fluctuating electromagnetic field . when the sensor is positioned within an environment and is subjected to a fluctuating electromagnetic field , the resonant frequency indicates the physical state of the environment .