This work addresses the limitations of conventional high-frequency methods in the modeling of lens antennas. While these structures are commonly analyzed using efficient Geometrical Optics (GO) / Physical Optics (PO) techniques, such approaches present significant limitations when the shadow region -defined as the surface area illuminated at incidence angles above the critical angle- is strongly illuminated, or when multilayer stratifications are incorporated. Furthermore, the full-wave validation is computationally intensive for medium-sized lenses, and impractical for electrically-large designs, leaving designers overly reliant on traditional methods and lacking detailed electromagnetic insight.

Firstly, to address the computational challenges with full-wave simulation of large lens designs, an approach based on PPW-embedding (Parallel-Plate Waveguide embedding) of the principal lens sections is followed. This reduction in problem size enables feasible full-wave simulations while also yielding valuable qualitative insights into the lens performance.

Secondly, to overcome the GO-PO limitations in predicting fields beyond the critical angle of incidence, this work follows a rigorous surface field modeling approach based on the Stratified-media Spectral Green’s Function, applied to locally flat surface approximations and evaluated using Inverse Fourier Transform in Steepest Descent Path (SDP) integration. The proposed method shows strong agreement with full-wave simulations and offers deeper insight into surface fields under critical-angle illumination, revealing their impact on radiation performance and enabling more effective lens designs.

Finally, the spectral approach is extended to the analysis of matching layers on the surfaces of high-permittivity lenses, demonstrating greater reliability than GO-PO techniques while maintaining computational efficiency superior to full-wave simulations. This study offers spectral-domain insight into surface lens phenomena and evaluates the performance enhancements provided by anti-reflection coatings. ... Read more

Accurate knowledge of the conductivity and permittivity of tissue is vital in the diagnosis of many diseases. Magnetic resonance electrical property tomography (MR-EPT) reconstructs these using measurements of the electromagnetic fields inside the MR-scanner. These properties
re reconstructed using two-dimensional contrast source inversion, due to a significant complexity reduction in comparison to its three-dimensional counterpart, allowing for reconstructions in a reasonable amount of time. Data acquisition is usually performed using a shielded
irdcage coil. Current methods, however, disregard the presence of this shielding, only accounting for it using a rough first-order approximation, as properly accounting for it would lead to an intractable computational load. This thesis shows a method of analytically describing this
hielding in the Greens functions and its efficient implementation in the corresponding Greens operators for CSI-EPT, exploiting the Greens functions being degenerate. This results in a significant increase in the reconstruction performance both qualitatively and quantitatively, at the cost of a slight increase of the computational load. The inversion problem being ill-posed leads to poor reconstruction performance on noisy data. This thesis presents a method for quantization of the tissue parameters, by enforcing a multi-modal distribution for these parameters.
through simulation, the quantization method shows a significant increase in performance for noisy data, at the cost of losing smaller details in the image. This improved model was tested on synthetic E-polarized data, and realistic three-dimensional data, showing a more robust, and
accurate reconstruction of the electrical properties for both. ... Read more

Electrical property tomography (EPT) reconstructs the human body’s conductivity and relative permittivity using the radio frequency field from a magnetic resonance machine. Since conductivity and relative permittivity can be biomarkers for many illnesses, it is necessary to reconstruct them accurately. To this end, this thesis focuses on the two-dimensional tranceive phase corrected contrast source inversion algorithm (2D-CSI).

This 2D-CSI algorithm is significantly faster than its three-dimensional (3D) counterpart. However, reconstruction artefacts may appear since the 2D field description is not always applicable. In this thesis, these artefacts are identified and it is investigated if the 2D-CSI algorithm can be improved to handle them. In particular, the proposed updates are, first, implementing the early stopping principle, then adding a positivity constraint to the conductivity and relative permittivity, and lastly, initialising with Helmholtz electrical property tomography (HEPT). The first two enhancements create a more robust algorithm, while the third update could be useful only in specific cases (with no fine structures and low noise levels).

After achieving a more robust algorithm, the 2D-CSI algorithm is applied to the realistic 3D dataset ”A Database for MR-based Electrical Properties Tomography” (ADEPT). The first and foremost challenge is accurately simulating the incident field, as the reconstructions depend a lot on the incident fields. If the incident fields do not match perfectly, the algorithm can only produce satisfactory reconstruction results in a part of the reconstruction domain. The reconstructions are only partly accurate because the reconstructed electric fields converge to a line creating line artefacts. It is shown that small changes in the incident fields produce small changes in the position of the artefacts. Based on this observation, a reconstruction strategy has been developed in which a reconstruction without artefacts is produced by combining reconstruction results of multiple incident fields. Integrating all modifications of the 2D-CSI
algorithm in one single framework shows that the conductivity and relative permittivity of the brain in the middle of the birdcage coil can be reconstructed accurately ... Read more

The tissue electrical properties of conductivity and permittivity affect the interactions of electromagnetic fields in the body. These properties vary throughout the different tissues as the tissue structure and composition varies. In this thesis, medical imaging and diagnosis is used as primary example to motivate exploration of a novel regularization approach to an MRI-based electrical properties tomography (EPT) method.

Total variation (TV) regularization has been shown to perform noise reduction in the iterative Contrast Source Inversion EPT (CSI-EPT) method. The Jacobi matrix inversion regularization, an alternative to the known conjugate gradient formulation, is elaborated and applied to an E-polarized MRI fields scenario such that this thesis presents the Jacobi step regularized CSI-EPT.

The alternative regularization method outperforms the known regularization method in the reconstruction qualities of noise-suppression and edge-preservation in the simulated MRI experiments using a virtual body model. Further advancements are also described, such as multiple inner-iterations Jacobi regularization and an anatomical prior initialization of the contrast function. Important future research topics are the incorporation and evaluation of the Jacobi step regularization into more advanced CSI-EPT versions, which are the three-dimensional and transceive phase based algorithms to correct realistic MRI data. ... Read more

A rigorous model based on classic electromagnetism to characterize the thermal radiation of real ohmic media is presented in this thesis. This model explains the available energy due to thermal agitation inside ohmic material based on Johnson's theory of thermal noise in electric circuits. The field is expanded in a finite number of modes (degrees of freedom per unit of volume), which are all independent and orthogonal from each other and are eigenvectors of Maxwell's Equations. The minimum distance for two eigenvectors to be independent is found as half of the real effective wavelength in the medium, based on which an analytical expression of the total energy available by thermal agitation in the finite volume is given. Integrating Poynting vectors of sources over the entire object volume, an analytical expression to estimate the total spectral power radiated out by a real ohmic material body is derived, which does not utilize Planck's law of black body radiation as an intermediary. Finally, a measurement campaign is proposed aiming at providing accurate measurements of the thermal radiation from silicon samples of small dimensions in the mm and sub-mm wave range. ... Read more

Recently, there has been an increasing demand for security in public places. As a result, non-destructive and fast millimetre-wave and submillimetre-wave imaging systems have gained more and more attention.
This project is based on Concealed Objects Stand-off Real-Time Imaging for Security (CONSORTIS), which is a European next-generation airport security imaging radar system published in 2017. In this project, we will discuss the design of the lens antenna illuminated by a leaky wave waveguide antenna for a large format focal plane array with wide scanning capabilities in three typical cases. The Coherent Fourier Optics (CFO) and leaky wave antenna design methodologies are used in this project. The system will be analysed in reception mode and then validated in transmission mode. We have a very promising performance with an aperture efficiency of about 80% in the centre and 47% at the edge of the array with a shaped top and AR coating. The directivity of the antenna at the edge is about 50.2 dB. And the scan loss is about -2.3 dB, which means it can scan about 10,000 beams in total.
... Read more

In this thesis, the joint DOA-range estimation of stationary targets is investigated using multiple FMCW MIMOs with super-resolution capability. To address the low azimuth resolution problem of single small MIMO, a novel topology of array is used, which consists of multiple MIMOs arranged along the azimuth to increase azimuth resolution by extending the effective aperture size. According to such topology, signal models are formulated using FMCW waveform. To accurately model the scenarios, targets are considered as near-field objects for the system, but they are treated as far-field targets for each MIMO.

After formulating signal models, two algorithms are investigated and tested to localize targets in the observing domain. The generalized 2D-MUSIC algorithm is applicable for both multi-static and mono-static configurations of the system. The FBSS technique is used to tackle highly correlated signals. Though this algorithm provides super-high resolutions, it requires prior knowledge of the number of targets (model order). The performance would drop significantly by incorrect estimation of model order. To avoid this limitation, an augmented Lagrangian method is introduced for the first time to address the localization problem, which is named extended C-SALSA. This method casts target localization problem as a sparse representation problem, and then the problem is transferred from estimating targets' locations to the problem of sparse spectrum estimation. It utilizes variable splitting and augmented Lagrangian to handle objective functions. For both algorithms, with the accurate positions of sensors in the system, geometrical constraints of the system can be maintained by applying the same search grid to all virtual arrays, consequently, data association is avoided.

The feasibility of both proposed methods are analyzed with numerical simulations of point targets and electromagnetic simulations of an extended target. MATLAB simulation results demonstrate that the azimuth resolution is increased using multiple MIMOs with both proposed algorithms. Besides the resolution, the accuracy of the generalized 2D-MUSIC is also compared with the derived CRLB. Moreover, CRLB is used to analyze the potential accuracy for the estimation results of the mono-static configuration. In spite of the requirement of model order, the generalized 2D-MUSIC outperforms the extended C-SALCA for extended targets and is more robust for off-grid targets. ... Read more

The quest to design a realizable quantum computer is a dream for many for the past few decades. At the moment, one of the promising designs is the transmon qubit, which is a superconducting qubit is the focus of this work. In order to analyze and design different components of a quantum chip, an understanding of the circuitry is required. By varying the circuit parameters, one can design the system according to requirements by constructing the required physical geometry that incorporates the design parameters. In this thesis work, part of the focus was to understand the transmon qubit circuit parameters, extract the circuit parameters such as capacitances from a physical geometry, build analytic and numerical 3D FEM models. Using a circuit model of the qubit system, the interdependence of the resonators that connects qubits with each other can be studied. Their resonances play a role in the qubit Hamiltonian and can be estimated by constructing accurate simulation models. This problem statement has been dealt with in the thesis work by constructing a hybrid circuitry of standard circuit components and 3D FEM simulated qubit unit cells. The quality factor of the measurement readout resonator of a qubit system needs to be characterized in a qubit system in order to target the rate and resolution at which the measurements can be done. In order to do so, the circuit parameters that affect the quality factor needs to be investigated. This problem statement has also been treated in this work. ... Read more

Typical antenna arrays are designed such that the active element pattern is symmetric around the broadside direction. However, applications exist, for example in satellite communication, where a symmetric pattern is not needed or even unwanted. This angular selectivity can be achieved using asymmetric elements. However, it is known that for well sampled infinite arrays the asymmetry of the active element pattern disappears. Although designs of under-sampled antenna arrays achieving an asymmetric active element pattern have been presented in literature, the fundamental properties of this type of arrays in terms of radiation characteristics have not been investigated in detail. This thesis studies the asymmetry in the active element pattern of a finite linear array of asymmetric elements. To this end an in-house method of moments code is developed in Matlab to simulate tilted dipoles in free space and in the proximity of a ground plane. The dependency of the asymmetry of the active element pattern on the inter-element distance, the skew angle of the elements and the number of elements in the array is analyzed and design rules are derived. Using entire domain basis functions, closed form expressions for spectral integrals and the periodicity of the array the implemented code enables the simulation of large arrays in a much shorter time compared to commercially available software, such as CST.

Regarding the choice of antenna element, a dipole bent into a Z-shape is proposed as an alternative for a tilted dipole. This type of dipole can be defined to have an equivalent radiation pattern to that of a tilted dipole. This shape of dipole can be implemented using standard PCB technology using horizontal metal strips and vertical vias. The Z-shaped dipoles are analyzed using a method of moments code based on horizontal and vertical dipoles. The spectral Green's function of stratified media can be included in the spectral domain expressions to account for the presence of dielectric slabs in realistic designs. ... Read more

A system has been developed that utilises the techniques of Ultra Wideband and Synthetic Aperture Radar to produce top view images of a scene using measurements from the side. The system consists of the PulsON P410 radar module, a set of antennas, a moving platform and an imaging algorithm. This thesis will cover all the aspects of the data acquisition part of the system with additionally an analysis on antennas. ... Read more