Date of Award

1-1-2011

Document Type

Campus Access Dissertation

Department

Electrical Engineering

First Advisor

Mohammod Ali

Abstract

he civil transportation infrastructure of structural concrete bridge is aging and deteriorating - principally as a result of the corrosion of the steel reinforcement that gives these structures their tensile strength. Assessing the condition of steel reinforcement inside the concrete is not simple, since the steel is typically buried beneath one inch or more of concrete. The strain sensor can be installed or embedded inside concrete during the construction phase of the infrastructures. Compared with expensive wired embedded sensors, wireless embedded sensors are easily deployable, low-cost, and can monitor in real time to minimize maintenance and inspection cycles. The built-in batteries in wireless embedded sensors need to be charged inductively. The proposed wireless embedded sensor node has two antennas, one for wireless communication to transmit data of sensing and controls at 2.45 GHz, the other for wireless power transfer to charge the built-in battery at 13.56 MHz from outside concrete periodically through near-field transformer-like magnetic coupling. These antennas are our research area. Firstly, from discrete measured data of dielectric constant and conductivity published in literatures, we developed the formula to extend their using range to all continuous available moisture content and frequency ranges. These formulas would be used as concrete parameters in our antenna simulation and to calculate the total microwave power loss of a plane wave penetrating concrete. These also give a good microwave theory foundation for the design and analysis of the antenna used inside concrete.

Secondly, a dipole, a loop, a microstrip patch and a PIFA were designed and analyzed for embedded operation inside concrete. Through antennas have been deeply studied in free space, there is little literature about their performance inside concrete environment. We studied their characteristics inside concrete of different moisture contents with or without presence of steel reinforcement. The existence the steel rebar in reinforced concrete deteriorates the dipole return loss significantly. It is found that the PIFA can be designed working efficiently inside concrete and satisfy transmission requirement for wireless embedded sensors with smaller (more compact) size than patch.

Finally, the loop antennas, acting like a loose transformer in near-field distance, rely on inductive (magnetic) coupling between two antennas, rather than on radiation of fields (electromagnetic waves) as in traditional far-field system. Little research has been made in near-filed antennas, especially in concrete. We designed a pair of 16-turn square spiral loop antennas operating inside concrete to maximize the inductive transmission between them at 13.56 MHz. The effect of concrete environments on perform of the loop antennas operating was investigated. All results are verified with simulation using Ansoft HFSS as well as measurement using 16-turn square spiral loop antennas fabricated on printed circuit boards (PCB) put inside concrete blocks.

It is shown that designed antennas satisfy the requirement of communication and power transferring of wireless sensors embedded inside concrete for civil infrastructure detection and monitoring.

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