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. / February 2017

The objective of this thesis is to design and implement a rapid, reconfigurable and portable reader for wirelessly interrogating inductively coupled passive sensors. While the current method of impedance analyzer is sensitive and an accurate, the instruments used are bulky and slow, substantially hampering in-field testing and interrogation of sensors. Current methods cannot provide a quantifiable measure on minimum necessary read-speeds and instrument accuracy desirable for rapid sensing applications. This work summarizes the design and hardware implementation of two reader methods that address the aforementioned requirements. Both reader methods are based on a reflectometer approach: Swept-frequency Reflectometer Reader and Switched-frequency Interrogation Technique (SWIFT). The first method is a much faster alternative to in-lab and in-field testing for structural health monitoring, and is intended as an immediate replacement for the impedance analyzer method. Switched-frequency Interrogation is specifically designed to satisfy the need for rapid and accurate reading, potentially for in-motion sensing applications. This method provides a way of empirically relating minimum necessary read-time required for desired read-ranges. It also facilitates quantification of uncertainty in measurements, which is very critical in determining instrument accuracy in-field. The system design and implementation of both methods are described in detail and experimental results are presented to benchmark the performance of the readers. Issues of instrument reliability and practical limitations are also discussed, with potential solutions. Both methods are intended as universal techniques for wirelessly interrogating coupled passive sensors, not limited to their current form of implementation. / text