THESIS
2023
1 online resource (xviii, 102 pages) : color illustrations
Abstract
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...[
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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.
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