Passive wireless resonator sensor for the measurement of AC electric field
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A passive wireless sensor is designed, fabricated and tested for the measurement of AC electric field in the vicinity of high voltage apparatus. This sensor is applicable in remote condition monitoring of high voltage apparatus where close distance measurements raises safety hazards for operators. The sensor is designed using a coaxial cavity resonator structure (in TEM mode) capacitively coupled to varactors. The resonance frequency of the sensor shifts corresponding to the capacitance variation of the varactors which in turn is perturbed by the external electric field. The electric field surrounding the apparatus induces a bias voltage over the terminals of the varactors. Therefore, the resonance frequency changes proportional to the inducing external electric field and correspondingly to the medium/high voltage. A printed circuit board on the top of the cavity provides coupling between the cavity and varactors and also between the varactors and the external field produced by the high voltage apparatus. The sensor structure is designed to resonate in the range of 2.4 GHz to 2.5 GHz of the industrial, scientific and medical (ISM) radio frequency band. A remote interrogation system identifies the instantaneous resonance frequency of the sensor by transmitting pulses of radio frequency (RF) signal and recording the ring back of the resonator. The ring back is down converted and analyzed to determine the resonance frequency of the sensor. Two possible applications of the sensor, i.e. voltage measurement and defect detection of insulators, are demonstrated by experimental results.