Browsing by Subject "Microwave imaging"

A 3D microwave imaging system suitable for clinical trials has been developed. The anatomy, histology, and pathology of breast cancer were all carefully considered in the development of this system. The central component of this system is a breast imaging chamber with an integrated 3D antenna array containing 36 custom designed bowtie patch antennas that radiate efficiently into human breast tissue. 3D full-wave finite element method models of this imaging chamber, complete with full antenna geometry, have been developed using Ansoft HFSS and verified experimentally. In addition, an electronic switching system using Gallium Arsenide (GaAs) absorptive RF multiplexer chips, a custom hardware control system with a parallel port interface utilizing TTL logic, and a custom software package with graphical user interface using Java and LabVIEW have all been developed. Finally, modeling of the breast (both healthy and malignant) was done using published data of the dielectric properties of human tissue, confirming the feasibility of cancer detection using this system.

A waveguide-fed metasurface consists of an array of metamaterial elements excited by a guided mode. When the metamaterial elements are excited, they in turn leak out a portion of the energy traveling through the waveguide to free space. As such, a waveguide-fed metasurface acts as an antenna. These antennas possess a planar form factor that offers tremendous dexterity in forming prescribed radiation patterns; a capability that has led to revolutionary advances in antenna engineering, microwave imaging, flat optics, among others.

Yet, the common approach to model and design such metasurfaces relies on effective surface properties, a methodology that is inspired by initial metamaterial designs. This methodology is only applicable to periodic arrangements of elements, and the assumption that the neighboring elements are identical. In the scenarios where the metasurface consists of an aperiodic array, or the neighboring elements are significantly different, or the coupling to the waveguide structure changes; the aforementioned approaches cannot predict the electromagnetic response of the waveguide-fed metasurface. In this thesis, I have implemented a robust technique to model waveguide-fed metasurfaces without any assumption on the metamaterial elements' geometry or arrangement. The only assumption is that the metamaterial elements can be modeled as effective dipoles, which is usually the case given the subwavelength size of metamaterial elements.

Throughout this document, the simulation tool will be referred to Dipole Model. In this framework, the total response of each dipole, representing a metamaterial element, depends on the mutual interaction between elements, as well as the perturbation of the guided mode. Both effects are taken into account and, by using full-wave simulations, I have confirmed the validity of the model and the ability to predict radiation patterns that can be used for beamforming as well as for microwave imaging.

Once the capabilities of the dipole model are compared with full wave simulations of both traditional antenna designs as well as more elaborated waveguide-fed metasurfaces, I develop an analysis on the use of these metasurfaces for microwave imaging systems. These systems are used to form images of buried objects, which is crucial in security screening and synthetic aperture radar (SAR). Traditionally, the hardware needed for many imaging techniques is cumbersome, including large arrays of antennas or bulky, moving parts. However, one attractive alternative to overcome these problems is to use dynamic metasurface antennas. By quickly varying the radiation patterns generated by these antennas, enough diverse measurements can be made in order to produce high quality images in a fraction of the time.

The compact size and speed come with a trade-off: a computationally intensive optical inverse problem has to be solved, which has so far prohibited these antennas from enjoying widespread use. I address this problem by reformulating the problem to make it similar to a SAR scenario, for which fast image reconstruction algorithms already exist. By adapting an algorithm known as the Range Migration to be compatible with these metasurfaces, I can cut down on real-time computation significantly. The computer simulations performed are highly promising for the field of microwave imaging, since it is demonstrated that diffraction-limited images can be acquired in a fraction of the time, in comparison with other imaging techniques.

Microwave imaging platforms conventionally take the form of antenna arrays or synthetic apertures. Inspired by methods in the optical regime, computational microwave imaging has recently taken hold as an alternative approach that uses spatially-diverse waveforms to multiplex scene information. In this dissertation, we use dynamic metasurface apertures to demonstrate improved hardware characteristics and capabilities in computational microwave imaging systems. In particular, we demonstrate waveguide-fed and cavity-backed dynamic metasurface apertures. A waveguide-fed dynamic metasurface aperture consists of a waveguide device loaded with numerous independently tunable metamaterial elements, each of which couples energy from the guided mode into a reconfigurable radiation pattern. We explicate design considerations for a waveguide-fed dynamic metasurface aperture, optimize its usage, and utilize it in computational imaging. In addition, we leverage the dynamic aperture's agility to demonstrate through-wall imaging and beamforming for synthetic aperture radar. Significant attention is also devoted to imaging with a single frequency, an approach which can ease the complexity and improve the performance of the required RF components.

Expanding on the waveguide-fed instantiation, we investigate cavity-backed dynamic apertures. These apertures employ disordered cavity modes to feed a multitude of radiating elements. We investigate this approach with two structures: a volumetric cavity, where we tune the boundary condition, and a planar PCB-based cavity, where the radiating elements are tuned. Capable of generating diverse radiation patterns, we use these structures to assess the utility of dynamic tuning in computational imaging systems. Many of the architectures studied in this dissertation chart a path toward a low-cost dynamic aperture with a favorable form factor, a platform which provides immense control over its emitted fields for a variety of microwave applications.

Synthetic aperture radar offers unparalleled satellite imaging capabilities for planetary observation. Future systems will realize high resolution with near-real-time revisit rates by using coordinated satellites, but their development has been slowed by the high cost, high power draw, and substantial weight associated with existing antenna technology. Metasurface antennas - a lightweight, low cost, and planar alternative - can address these challenges to make large scale, multi-satellite systems practical. In this work, an electronically steered metasurface antenna prototype is developed for synthetic aperture imaging. A cohesive approach to modeling and design led to a Nyquist sampled layout which minimizes inter-element coupling and suppresses grating lobes. Experimental measurements validate its ability to steer a beam in 2D across a wide bandwidth. Robust performance and favorable hardware characteristics have poised metasurface antennas to affect many microwave industries and to facilitate multi-satellite constellations for spaceborne synthetic aperture radar.

Microwave imaging is an important tool for security screening applications. Low-power microwave radiation is used to safely and noninvasively form high resolution images of people to screen for concealed threat objects. This is because microwaves easily pass through clothing and strongly reflect off skin and many materials of interest.

Image resolution improves with large aperture size and bandwidth. Commercial imaging systems realize large apertures two ways; as phased array of antennas, or synthetically by mechanically scanning antennas. In both cases long acquisition times permit only one person to be screened at a time while holding a pose. Security checkpoints employing these systems suffer low screening throughput and present a bottleneck that endangers people. Economically increasing screening throughput requires allowing people to move unimpeded while being imaged.

Computational imaging with a frequency diverse aperture provides a path forward. Frequency diverse apertures are composed of antennas designed to have spatially uncorrelated radiation patterns as a function of frequency. A transceiver drives the antennas with a frequency sweep to rapidly take uncorrelated measurements of a scene. A physical model relating transceiver measurements to scene reflectivity is then numerically solved to form an image. In this way hardware complexity is traded for modeling complexity, leveraging computing technology. The resulting system is inexpensive, modular, flat, and has no moving parts.

An experimental microwave imaging system consisting of a frequency diverse aperture driven by a MIMO transceiver operating from 17.5 GHz to 26.5 GHz is described. The imaging system has 24 Tx antennas, 72 Rx antennas, and samples 101 frequency points giving 174528 possible measurement combinations. The transceiver uses an orthogonal coding strategy to acquire complete sets of measurements at 7 Hz, enabling a walk-while-scan modality. Depth cameras are integrated to inform image reconstruction and analysis. Several acceleration strategies are pursued to reduce image reconstruction times. A comprehensive simulation platform is used to optimize system configuration. Near real-time imaging of multiple people in motion is demonstrated.

Images of people walking present unique challenges for automated threat detection. A deformable stitching model for combining images is developed, and a framework for applying the stitching model is proposed.