Non-invasive near-field measurement setup based on modulated scatterer technique applied to microwave tomography

The main focus of this thesis is to address the design and development of a near-field (NF) imaging setup based on the modulated scatterer technique (MST). MST is a well-known approach used in applications where accurate and perturbation-free measurement results are necessary. Of the possible implementations available for making an MST probe, including electrical, optical and mechanical, the optically modulated scatterer OMS was considered in order to provide nearly perturbation-free measurement due to the invisibility of optical fiber to the radio-frequency electromagnetic fields. The OMS probe consists of a commercial, off-the-shelf (COTS) photodiode chip (nonlinear device), a short-dipole antenna acting as a scatterer and a matching network (passive circuit). The latter improves the scattering properties and also increases the sensitivity of the OMS probe within the frequency range in which the matching network is optimized. The radiation characteristics of the probe, including cross-polarization response and omnidirectional sensitivity, were both theoretically and experimentally investigated. Finally, the performance and reliability of the probe was studied by comparing measured near-field distributions on a known field distribution with simulations.;Dynamic range and linearity of the developed NF imager was improved by adding a carrier canceller circuit to the front-end of the receiver. The canceller eliminates the carrier on which no information is transmitted and leaves the sidebands intact. This enables us to increase the amplification gain to achieve better signal-to-noise ratio (SNR) and more importantly to expand the imager's dynamic range. The carrier at the receiving port is minimized by combining the received signal with a 180-degree out-of-phase canceller whose magnitude and phase are adjusted adaptively. We first examined the canceller performance, which leads to a reduction of about 60 dB in the magnitude of the carrier, and then studied its impact on the operation of the NF imager. Significant improvements of NF images were obtained in both dynamic range and linearity.;Increased imaging speed was obtained using an array of OMS probes, which reduces mechanical movements. Mutual-coupling, switching time and shadowing effect, which all may affect the performance of the array, were investigated. Then, the results obtained by the array were validated in a NF imager by measuring the E-field distribution of an antenna under test (AUT) and comparing it with a simulation. Calibration and data averaging were applied to raw data to compensate the probes for uncertainties in fabrication and interaction between array/AUT and array/receiving antenna.