Sharif Digital Repository

Novel Dirac Leaky Wave Antennas (DLWAs) based on Substrate Integrated Waveguide (SIW) are presented in this thesis, for millimeter wave frequencies. These DLWAs provide interesting features such as radiating through broadside, beam scanning and ease in fabrication that are well suited for emerging fifth generation (5G) and IoT applications. In this thesis, it is shown that a Dirac photonic crystal can be realized in SIW technology composed of air columns inside a host SIW waveguide, exhibiting a closed bandgap and linear dispersion around broadside. Phase and attenuation constants are controlled to obtain directive beam and scanning in a wide range of angles (from -30˚ to 20˚). This DLWA... 

In the last two decades, reconfigurable antennas have attracted significant research attention. Utilization of such antennas can lead to decrease in cost and power consumption in modern communication systems. The main focus of efforts is on frequency reconfigurability using different mechanisms while maintaining desirable antenna characteristics over the tuning range. In this thesis, our goal is to design, analyze and fabricate a reconfigurable antenna based on the concept of polarization standing wave (PSW) in the X band using only a simple physical tuning mechanism. Two anisotropic handedness preserving mirrors (AHPM) are placed at both ends of a cylindrical cavity resonator, thus... 

Expansion of communication and the emergence of new fields such as the Internet of Things, has led to the development of smart cars and especially self-driving cars. The antennas used in the radar of these cars are of special importance for a better understanding of the environment around the car. The important parameters for the design of antennae for automotive radar are: good resolution, narrow beam width, the ability to change the angle of the main beam, etc. Among the most common antennas used in these radars are directional antennas, for this reason phased array antennas and leaky wave antennas are suitable options for this application. In general, the complexity and difficulty in the... 

Structures with time-varying electromagnetic properties can potentially yield non-reciprocity, amplification, harmonic generation, and other interesting wave propagation phenomena. To date, some studies have dealt with time-varying media and circuits, including time-varying optical ring resonators having degenerate travelling wave resonances. In time-varying ring resonators, the clockwise and counter clockwise resonances lose their degeneracy, and non-reciprocity is observed. One of the main challenges of such research is the need to induce time-variations in the entirety of the resonator or structure, which makes their implementation very limited or impossible in practice. The purpose of... 

Antenna arrayss have long been utilized in many applications including communication, radar, and imaging. With the advancement of the telecommunication industry and the emergence of new requirements in the fifth generation of telecommunications, imaging and the Internet of Things, new antenna arrays are designed at higher operating frequencies and with particular specifications. One of the major challenges in using these types of antennas is unwanted mutual coupling in these systems. These couplings can occur between the antenna elements of the array itself, or between two or more arrays that are adjacent to each other. These couplings can seriously affect the performance of the system.... 

Artificial neural networks are systems based on the brain’s functionality which in many cases are able to process highly complex computational tasks like speech recognition, image recognition ,and time series prediction. Due to the complexity of training algorithms in recurrent neural networks, reservoir computers have significant importance in machine learning.Due to low power consumption and crosstalk, high bandwidth and high-speed computing in optics, reservoir computing has proceeded to optical implementations. A reservoir computer consists of three layers: the input layer, reservoir, and output layer. A recurrent neural network is usually used as the reservoir in reservoir computers.... 

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... 

In this paper our purpose is introducing new method for designing dual pass band filters that it can make the tuning steps easier and more determinant. This method based on matrix coupling theory. We assign a specific arrange for desired coupling matrix that it helps to our porpuse. After design any effect of matrix element has been apparent, so tune steps can be more routine. we used half cut cylinder resonators for designing dual band filter that it decrease size and mass of the filter intensily.After designing dual band filter we can transform its out put to achieve diplexer. New diplexer method also has been introduced at the end of paper  

Researchers have long considered the phenomenon of parametric amplification. This phenomenon is introduced first in nonlinear structures and then in limited systems of time-varying linear structures. Time-modulated systems have many capabilities, such as amplification, non-reciprocity, and other phenomena in wave propagation. So far, time-varying circuits have been studied in terms of stability and analytical solution. The purpose of this project is to investigate parametric amplification in time-varying linear systems. Studying a structure under a circuit model is essential to other analyses practically implemented in various frequency ranges from microwave to light. As a result, this... 

Electronic processors have encountered Moore’s limitations and they can not meet expectations following computer science advances in new algorithms such as neural networks. These limitations have brought research to focus on light as a high-capacity carrier of information. Light’s degrees of freedom can be exploited whenever the structure has devices able to carry, keep, manipulate and detect each DoF's information but this exploitation may itself lead to either a large footprint or more power consumption. Moreover, there are some scientific and technological difficulties in matching different computational algorithms to these DoF’s requirements. As a result, one must consider all advantages... 

Early detection of diseases has always been critical, especially during the outbreak of infectious and epidemic diseases plays an essential role in controlling the spread of the disease. Finding diagnostic methods with high sensitivity, especially in the early stages of the disease, is an important challenge in the face of new diseases. periodic structures biosensors are accurate sensors and have been very popular in recent years. In this thesis, two types of sensors that are very sensitive are investigated using a circuit model. The first structure is the array of sub-wavelength apertures perforated on a PEC film. In these structures, the extraordinary transmission phenomena are used for... 

This thesis formulates Green's function of periodic arrays of two-dimensional rectangular holes Perforated on a PEC film. Green's function of two-dimensional periodic free space in the spectrum domain is computed. Using Floquet theory and considering the holes as rectangular waveguides, Green's function of the structure is presented.For sub-wavelength holes in cut-off and propagating cases, numerical results are presented. The effects of changing the aperture's thickness and the number of required modes have been reported. The structure under a normal and oblique incident wave is investigated, and the high conformity of the presented model results and full-wave simulations is observed.In... 

Epsilon-near-zero media have attracted considerable attention in the last decade due to their exceptional behavior in wave propagation. The purpose of this research project is to analyze time-periodic ENZ structures. To this end, layered structures consisting of plasmonic metal and dielectric layers are used to achieve the epsilon-near-zero effect in a specific frequency range, and by time-variations of the dielectric layers, an effective time-varying (TV) ENZ is achieved. A method based on transfer matrix method is developed to analyze such structures while they are undergoing time-variations, and primarily under normal incident waves. Various structures based on such TV layered structures... 

We study the phenomenon of parametric amplification and instability in the context of time-varying electromagnetic structures, particularly dielectric slabs with periodically modulated permittivity. These structures show particular promise inasmuch as they are capable of very large amplifications when illuminated by an electromagnetic wave of half the modulation frequency. Successive studies have corroborated this finding but none have yet been able to ascertain the nature of amplification in such devices. On top of that, the problem of instability in time-periodic slabs has either been disregarded altogether or it has been subjected to loose analysis, leading to mere speculations which are... 

This thesis presents a resonant time-periodic structure based on the Fabry-Perot resonator for frequency comb generation. Time-periodic structures cannot generate a frequency comb by themselves because side-harmonic amplitudes are really weak. However, due to resonant structure, it is expected that the side-harmonics, which are generated because of the time-periodic part of the structure, are increased if they are close to the main modes of the structure. But the results show that, generally, the amplitude of side harmonics, which are close to the main mode, is increased and far side-harmonics don't have remarkable amplitudes. However, we will show that if the modulation frequency of the... 

Today's millimeter imaging has been widely used in the field of security and medicine. With increasing security threats, there is a need for improved security equipment. Millimetric waves not only have the ability to penetrate dielectric materials, but also the ability to separate them to detect hazardous materials. Also, the millimeter-field imagery is capable of detecting different materials from each other, which is an advantage in detecting hidden cases. Unlike X-ray imaging, millimeter waves do not have the ability to ionize the cells of the body, which reduces the risk of creating radios for imaging. In this project, an array antenna with two orthogonal poles is designed. Macrostrip... 

The Emergence of modern wireless communication standards and the need to use the frequency spectrum efficiently has made communication system designers exert complex modulation schemes. One of the results of such designs that has a direct impact on the design procedure of power amplifiers is that there will be signals with high peak to average power ratio.The obvious consequence of this manner in the design of communication systems would be the reduction of power amplifier efficiency due to its operation in its back-off region. The solution will be to use load modulated power amplifiers such as Doherty amplifier which brings up the problem of limited bandwidth.In this thesis, we tackle this... 

Complex waves possess complex propagation constants even in the absence of losses. Such waves may be excited in open- or closed-guided wave structures such as inhomogeneously or anisotropically filled waveguides, coupled right-handed/left-handed transmission lines, and periodic structures. In the absence of losses, complex modes appear in pairs with complex conjugate propagation constants. In open-guided wave configurations, complex modes will be of either the proper or improper (leaky wave) type. Proper complex modes are accompanied by an electromagnetic field that decays exponentially in the transverse direction, away from the waveguide. By contrast, electromagnetic fields associated with... 

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...