Sharif Digital Repository

Time-varying media have been in center of attention since recent years for their plethora of applications. Achieving non-reciprocity, one-way propagation, frequency modulation, and signal amplification are some applications of such media which are accessible from microwave to optical frequencies. The aim of this thesis is to first find a method for the analysis of wave propagation in homogeneous time-varying media. To this end, a new formulation is presented based on differential transfer matrices enabling us to find amplitudes, average power, and average energy of a wave in a homogeneous time-varying medium for arbitrary temporal variation of permittivity. In addition,this formulation... 

Nonuniform superconducting microstrip transmission lines in microwave regime have been investigated. Nonuniform transmission lines provide the capability of controlling line parameters. Photonic structures with subwavelength features can be homogenized and thus be accurately approximated by homogeneous yet spatially dispersive structures. This idea is here applied to nonuniform superconducting transmission lines with subwavelength nonuniformities, i.e. subcentimeter features in the microwave regime. This modelling is quite useful in geometrical and optical control of quasi-TEM wave propagation and dispersion engineering along microwave superconducting lines. This approach has applications... 

In this thesis, we have investigated nonuniform normal and superconducting microstrip transmission lines in microwave regime. Photonic structures with subwavelength features can be homogenized and thus be accurately approximated by homogeneous structures. This idea is here applied to nonuniform transmission lines with subwavelength nonuniformities, i.e. subcentimeter features in the microwave regime. We present an analytical model based on telegraphers’ equations in order to obtain line propagation characteristics; characteristic impedance, propagation constant, and group velocity. The accuracy of analysis in normal and superconducting lines is evaluated by measurement and electromagnetic... 

Recent computational and technological progresses in millimeter-wave (MMW) band provide a high promising opportunity of various applications. The possibility of high rate data transferring is of strong interest for indoor wireless communications. Ability of high resolution and non-invasive imaging attract so much interest in medical and surveillance imaging and radar systems. In all of mentioned applications, human body plays an important role. Human body blockage (HBB) and human body shadowing (HBS) in wireless communications; concealed weapon detection (CWD) and detection of contraband objects on the surface of human body; and detection of human body presence and movement in buildings by... 

Due to the gyrotropic properties of ferrites, they have the ability to create nonreciprocal structures and have been used in microwave frequencies for a long time. Despite the use of these magnetic materials in ferrite devices, circulators, antennas, insulators, etc. The investigation of wave scattering by ferrite materials has been very limited and has only been limited to finding the radar cross-section. On the other hand, metasurfaes have been the subject of many studies in recent years in order to control propagation waves and create unusual scattering (reflection or propagation) from a surface.In this thesis, electromagnetic wave scattering by a grounded ferrite disk and the phenomena... 

Recently, there has been a great interest toward site–specific modeling of outdoor radio wave propagation. Wave propagation in very large environments compared to a wavelength builds up an important category of electromagnetic wave propagation problems which occurs in
versatile criteria from optical devices to long wireless microwave links. Well-known methods such as different ray tracing schemes used in these problems suffer from long execution time and great amount of needed memory. A new method has been proposed in this thesis to overcome such difficulties for high frequency wave propagation in large scale problems. The proposed method is based on tracing 3D vectorial Gaussian beam.... 

In this project, different types of optical frequency selector devices are compared with each other and the factors limiting their behavior (losses, throughput, quality factor, etc.) are precisely analyzed. Change this behavior in near-field and far-field scales as well as the behavior of these devices in nano-dimensions are analyzed. Also, the main theoretical and technological limitations of different types of optical filters in far-field and near-field regimes are compared. In the following, the scaling behavior of different filter families in passing to nanometric dimensions is investigated and finally the limitations of the possibility of making the selected sample are analyzed. For... 

The electrons scatter by the impurities during propagation through the media and become localized by increasing the impurities. So the metal insulator transition occurs. This phenomenon was first proposed by W. Anderson and is known as Anderson Localization. In this thesis the Anderson localization of electromagnetic waves is investigated. The disordered media with random dielectric constant is studied by considering Maxwell’s equations. There are different numerical methods to study this phenomenon such as transfer matrix method and level statistics we have used in this thesis. In three dimension, different scaling behavior shows the phase transition from localized states to extended states... 

This thesis explores the feasibility of implementing terahertz wireless communication in indoor environments (as suggested in 6G) by modeling the propagation of terahertz band waves. The roughness of indoor surfaces at the terahertz frequencies is investigated through the detailed examination of the electromagnetic fields scattered from these surfaces using the method of moments (MoM). To account for the random roughness of these surfaces, the Monte Carlo method is utilized, generating surfaces with Gaussian statistical distribution, and obtaining the scattered electromagnetic fields using MoM. The final results represent an average of all simulated scenarios. In addition, the study... 

The purpose of this study is to estimate the temperature distribution of the internal body organs using electromagnetic waves. When the external electromagnetic field propagates in biological tissues under a process like microwave hyperthermia, the temperature of the tissue at the field concentration area increases. Due to temperature and frequency dependence of the tissue dielectric, the tissue permittivity changes; thus, the electric field scattered by the tissue changes. Therefore, by measuring the field scattered from the object, dielectric changes of the tissue structure can be calculated. Since the biological tissues’ dielectric coefficient at frequencies lower than 2GHz is linear,... 

In this research, an electromagnetic wave absorber based on metamaterial structures is designed and built that is able to absorb a significant part of the wave power (up to 90%) in the millimeter wave band. The characteristics of this absorber are very low thickness (about a quarter of the wavelength compared to the central frequency) and high absorption bandwidth (up to 50% compared to the central frequency). Among the various structures studied, the structure of mesh arrays or conductive loops was selected for design. Using the Babinet principle, a transmission line model was proposed to analyze the behavior of this structure and became the basis for the design of the electromagnetic... 

In this dissertation, hybrid meshless method for electromagnetic wave scattering has been presented. The idea of the dissertation is the use of two robust meshless shape functions i.e. MLS and RPIM simultaneously to solve differential and integral equations from 3D scattering equations respectively. Finite element method (FEM) and method of moments (MoM) are used extremely for computing scattering problems. These methods are powerful and their results are accurate enough. One of their disadvantages is the need of mesh generation. Mesh generation in FEM is a time consuming process. Furthermore, in electromagnetic problems that involve geometrical, the use of an underlying mesh creates... 

In this thesis, we propose and discuss efficient GPU implementation using CUDA for simulating electromagnetic scattering. We use SPH as a meshless particle method to electromagnetic transient simulation in time domain. Smoothed particle hydrodynamics (SPH) has been recently reformulated by the authors, and implemented in the so-called smoothed particle electromagnetics (SPEM) method. In SPEM two set of electric and magnetic staggered particles have to be generated. These particles are the points in which the field components are computed at each time step by using the information belonging to the neighboring ones.
On the other hand, CUDA™ is a general purpose parallel computing platform... 

In recent years, there has been a great studying on site-specific indoor wave propagation mechanism. Conventional ray tracing method is the most common way to predict the profile of the signal in these environments. This method provides us with a deep understanding of signal propagation mechanism including reflection, refraction and diffraction. Knowing these characteristics, we can design a wireless system with optimal performance. Modified Wavefrond Decomposition Method is a ray tracing speed up technique. In this method, a bundle of rays will be traced instead of tracing single rays. If a ray bundle illuminates the receiver location, a ray that exactly crosses the receiver point would be... 

In this dissertation, we design and simulate Electromagnetic (EM) wave absorbers using periodic array of ultra-thin metallic ribbons, metallic disks and graphene disks based on circuit theory. The equivalent circuit for EM structures significantly eliminates the requirement of systems with advanced hardwares. Firstly, we categorize the metallic absorbers in the literature in two types: narrowband absorbers and broadband absorbers. Surface plasmon polariton, magnetic resonance, interference theory and localized surface plasmon are employed to illustrate the mechanism behind the narrowband absorption. Therefore, they will be the basis for classifying the narrowband absorbers in this work.... 

The main goal of this thesis is introducing a method to design optimum nanoantenna arrays for detection applications. The proposed method is based on the multiple-scattering T-matrix in connection with the Ewald method. The formulation is systematic, quite general, easily traceable, and fast. Its high speed of analysis makes it well suited to design optimizations.In particular, we calculate multiple-scattering terms of the T-matrix formulation with the Ewald method. To the best of our knowledge no report on derivation of the T-matrix of a 3D (or even 2D) periodic array from the T-matrix of the isolated element, based on Ewald method has been reported before.Finally, a software has been...