Application of reconfigurable field perturbing elements in microwave imaging within a metallic enclosure
Ghasemi, Amir H
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One of the techniques to enhance the performance of microwave imaging (MWI) systems is to increase the amount of independent measured data that is collected when interrogating the object-of-interest (OI). A novel method is introduced which employs field perturbing elements (FPEs) to achieve this goal. The research reported herein utilizes a cylindrical metallic enclosure having an octagonal cross-section, i.e. with at-faceted walls on which a set of electrically controlled field perturbing elements (FPEs) that project into the chamber. The FPEs are implemented as printed circuit boards that include diodes that can be forward or reversed biased. Transmit and receive antennas are placed on the walls of the chamber and for each transmitter the interrogating eld can be changed by turning one or more of the recon figurable plates ON or OFF. This allows one to reduce the number of transmit antennas that are used. Our current system consists of an octogonally shaped chamber with four transmitter/receiver antennas. The size of the metallic chamber is on the order of a few wavelengths and with the entirety of the enclosure being made up of a conductive metallic material, it constitutes a quasi-resonant chamber (we utilize an open top adding some loss to the chamber). The fi eld perturbing elements include a pattern of segmented PCB traces with multiple diodes connecting the trace segments. To reduce the computational resources required to model these reconfi gurable plates, the design criteria is to choose a pattern of the segmented PCB traces that minimally perturb the fi eld within the chamber when the diodes are \OFF" (i.e. in reverse-biased mode) by choosing the frequencies that match closely with the numerical model. This allows us to remove the plates from the numerical model when in the \OFF" mode with small effects on the accuracy. In the \ON" mode, the plates are modelled as PEC surfaces. To evaluate the performance of the proposed approach, a number of imaging experiments were performed with a DUT placed in two positions in the chamber for different confi gurations and combinations of the electrically recon figurable plates. a comparison between two cases of FPE con figurations have been performed. First, the case with FPEs in OFF mode and second, when the FPEs are in ON mode in two different frequencies. The critera for selecting the frequencies are also described. The results of the experiments are presented in this thesis.