Microwaves can penetrate optically opaque materials without harmful ionizing effects, and thus provide distinct advantages over x-ray imaging in various fields, including security-screening, remote sensing, medical imaging, through-wall imaging, civil and industrial applications. However, conventional microwave imaging methods, such as synthetic aperture radar (SAR) or phased array methods, generally rely on mechanical or electrical scanning to collect the spatial data and the resulting imaging speed and complexity remain challenges for constructing microwave imaging systems.

In this thesis, I investigate the use of diversity techniques to develop high-speed, low-profile and low-cost microwave imaging systems. Firstly, I propose a single-shot frequency-diverse near-field imaging system...[ Read more ]

Microwaves can penetrate optically opaque materials without harmful ionizing effects, and thus provide distinct advantages over x-ray imaging in various fields, including security-screening, remote sensing, medical imaging, through-wall imaging, civil and industrial applications. However, conventional microwave imaging methods, such as synthetic aperture radar (SAR) or phased array methods, generally rely on mechanical or electrical scanning to collect the spatial data and the resulting imaging speed and complexity remain challenges for constructing microwave imaging systems.

In this thesis, I investigate the use of diversity techniques to develop high-speed, low-profile and low-cost microwave imaging systems. Firstly, I propose a single-shot frequency-diverse near-field imaging system by exploiting a frequency diversity technique based on high-scanning-rate leaky-wave antennas (LWAs). Frequency diversity is an all-electronic technique and can achieve data collection by sweeping frequency without mechanical moving parts or active switching circuit component. In addition, the potential performance of the LWA design is analyzed by introducing a figure of merit based on sensing capacity. It is revealed that a transceiver LWA with a high scanning rate is very effective for providing independent measurement modes. Analytical, simulation and experimental results are provided to characterize and demonstrate the proposed system and show that it can provide image reconstruction across narrow bandwidths.

Building on frequency diversity techniques, I investigate the use of high scanning-rate leaky-wave antennas in a multiple-input multiple-output (MIMO) configuration to achieve both frequency and spatial diversity in an imaging system. Compared with frequency-diversity only and spatial-diversity only imaging systems, the proposed system can provide enhanced imaging performance by leveraging both spatial and frequency diversity simultaneously. In addition, an extended Rytov approximation (xRA), recently shown to provide accurate reconstructions for high permittivity and electrically large sized low-loss objects, is also included in this approach. Numerical and experimental examples demonstrate that the proposed system with xRA can accurately estimate the contrast function amplitude and the positions of dielectric scatterers in an imaging region.

Apart from frequency and spatial diversity techniques, I investigate the use of pattern diversity for reducing the number of required measurement nodes in utilizing xRA for radio frequency (RF) imaging. For indoor RF imaging using radio tomographic imaging (RTI), 20-40 WiFi nodes are usually utilized around the imaging region. This implies that a conventional WiFi network must be supplemented with additional WiFi nodes specifically dedicated to the imaging application. The proposed approach is to exploit antenna pattern diversity so that each node can collect multiple independent measurements from the same measurement location, thereby decreasing the number of measurement nodes required. Simulation results are provided to verify the RF imaging approach with reduced measurement nodes, which demonstrates the potential of using pattern diversity.

In all the research contributions described, analyses, simulations and/or experimental results are utilized to demonstrate the effectiveness of my new and novel approaches to microwave imaging.