Investigation of embedded sensors and microstrip patch antennas for structural health monitoring

This thesis deals with the design and development of components of an embeddable passive wireless sensor used in structural health monitoring applications. The sensors are designed to operate in a variable environment (changeable permittivity epsilonr) with high electrical losses at depths of up to 100 mm. Sensors are passive devices consisting of a cavity resonator attached to a microstrip patch antenna. The attached antenna, is used to communicate the measured data, with an external data acquisition unit. Activation of the sensors is accomplished by an external interrogator unit located at the concrete surface.;The components of the passive sensor (cavity and antenna) are designed individually. As a first step, several different types of microwave cavity resonators are investigated. This involves extensive parametric study of each cavity using the 3D full wave software Microstripes. Several new tunable cavities are introduced along with measurement data.;The second part of the thesis focuses on design, simulation, and fabrication of embedded microstrip patch antennas. The embedded antennas provide the fundamental link with the external data acquisition units, and thus must be designed carefully. It is well known that embedding an antenna or placing any type of material on its surface will change its properties. This includes a shift in the resonant frequency and gain pattern distortion depending on the thickness and permittivity of the dielectric (concrete) slab. These issues are addressed, and discussed in this work. Gain and resonant properties of a microstrip patch antenna embedded within concrete are investigated and analyzed. Detailed study of the embedded patch antenna shows that due to extreme variations in the concrete permittivity and water content, embedding the antenna within the concrete can significantly deteriorate its gain and shift the resonant frequency. This problem is alleviated by the use of a cover layer on the antenna, which minimizes the concrete effect and reduces the antenna sensitivity to surrounding materials. A comparison of the measured far field patterns of the embedded patch antennas shows significant gain improvements of up to 25 dB at resonance for a patch with a cover layer. Additionally, resonant frequency stabilization is achieved, which is essential for the operation of the sensor system. Finally, an interrogator is used to remotely detect the resonance of the wireless embedded sensor. This is accomplished successfully with the data presented in the last section.