Sharif Digital Repository

So far, investigations have shown that open dielectric resonator antennas (DRAs) offer several advantages at microwave and millimeter wave frequencies. These advantages include small size, large impedance bandwidth, high radiation efficiency, ease of manufacturing and simple feed-coupling mechanisms (such as probe coupling, aperture coupling or proximity coupling to micro-strip lines). Most microwave ferrites have dielectric constants in the desirable range and they can be used in place of dielectric materials. Such ferrite resonator antennas (FRAs) offer a potential advantage in that their permeability tensor can be controlled by applying an external magnetic field. Up to now and by the... 

Today as various electronic and telecommunication devices like cell phones and PDAs are being popularized, MMIC technology is gaining a significant importance. One of the priorities of this technology is trying to miniaturize these popular devices. But reducing size of passive microwave components used in these devices requires microwave wavelength to decrease.Components fabricated by conventional deposition techniques aren’t thick enough to reduce the wavelength, so permittivities much higher than those of conventional dielectrics could be a possible solution. Artificial dielectric layers (ADL) can provide such high permittivities.So utilizing these layers can possibly reduce the size of... 

So far, investigations have shown that open dielectricresonator antennas(DRAs) offer several advantages at microwaveand millimeter wave frequencies and make them appropriate in low directional-gain applications . These advantages include small size, large impedance bandwidth, high radiation efficiency, ease of manufacturing and simple feed-coupling mechanisms (such as probe coupling, aperture coupling or proximity coupling to micro-strip lines). Most microwave ferrites have dielectric constants in the desirable range and they can be used in place of dielectric materials. Such ferrite resonator antennas (FRAs) offer a potential advantage inthat their permeability tensor can be controlled by... 

In wireless communication systems increasing the gain of antennas is seminal to increase in coverage and data transmission rate, and reducing the possible errors. One of the main factors for improving the antenna gain is matching the polarizations of the transmitter and the receiver antennas. Towards this end, both the transmitter and the receiver antennas should have the same axial ratio, specific direction and polarizations. As far as the flexibility of the direction angle between the transmitter and the receiver is concerned, the so-called circular polarization is preferred do the linear polarization. The antennas with circular polarizations are prominently utilized in satellites,... 

The phenomenon of extraordinary transmission of electromagnetic waves through arrays of small holesin metallic plates has attracted the attention of many researchers in the past decade.This behavior may be utilized to fabricate extremely narrowband microwave or optical filters and near-field sensing devices where the sub-millimeter hole can act like a probe. Despite almost ten years of continuous research, however, the uncertainty regarding the mechanisms underlying this phenomenon somewhat persists. Different theories concerning the origin of such behaviors, including surface waves, diffraction of electromagnetic waves, and waveguide phenomenon inside holes and slits have been proposed. But... 

The objective of this thesis is to design an electrically tunable array of rectangular waveguide slot antennas. Because of its simplicity in design and fan shaped radiation pattern, for many decades, this kind of antennas were used in many systems like radars. Conventionally, beam steering of these types of antennas is done by a frequency sweep of waveguide in vicinity of the center frequency. Because each element is a resonant slot, its low bandwidth puts limitations in the frequency sweep. In this project, instead of frequency sweep in slot array antennas, we will consider adjusting these antennas using varactor diodes. Apart from varactor’s adjusting capability and its effect on radiated... 

Millimeter wave technology may prove to be one of the key technologies of the 21st century, covering a broad range of applications including high-speed telecommunication, wireless sensing, and ultra-fast digital computing. The ultimate (commercial) success of these technologies depends on the ability to integrate mm-wave circuitry on a chip, bringing about significant size and cost reduction. Traditionally, mm-wave systems have made extensive use of waveguides based on hollow- or dielectric-filled metallic cavities for the transfer and processing of signals. Cavity waveguides exhibit very low loss, but are not well-suited to high volume manufacturing or on-chip integration due to their... 

Artificial periodic mediums such as metamaterials or photonic crystals have been intensively studied in recent years due to their unique characteristics and the ability to control their effective electromagnetic parameters. Developing accurate methods for the extraction of these parameters is highly desirable, given the potential application of artificial materials in optical and microwave devices. In this thesis a new method is presented for computing the effective bulk and surface parameters of three-dimensional artificial periodic materials. The effective, bulk permeability and permittivity tensors are shown to be directly related to the eigenvectors of the generalized transfer matrix of... 

In its most general form, analysis of macroscopic electromagnetic behaviour of a medium must include the dependence of magnetic flux density on the electric field and electric flux density on the magnetic filed. This new degree of freedom can lead to new propagation phenomena and result in novel microwave and optical devices. In recent years, numerous theoritical and exprimental studies have been carried out to fabricate these bi-anisotropic materials and understand their behaviour. A particular class of bi-anisotropic materials is electromagnetically equivalent to an anisotropic medium with gyrotropic permittivity and permeability tensors that operate on pseudo-electromagnetic fields. In... 

Surface plasmons are the result of a resonant interaction between the waves and the mobile electrons at the interface between negative and positive permittivity materials stimulated by incident light. Due to their specific quantum mechanical nature and zero electrical resistance, superconductors are capable of supporting low-loss plasma waves with extremely large propagation length. In this study, palsmon confinement in some superconducting waveguide structures has been measured by computing the effective index parameter. Superconductors are classified in to high-Tc an low-Tc categories. It is not possible for plasma waves to propagate in low-Tc superconductors, because the plasma energy... 

Metamaterials are engineered sub-wavelength structures which show electromagnetic properties not found in natural materials. There is a special interest in superconductor metamaterials in microwave engineering. These metamaterials have low loss, are compact and show useful properties like tunability and parametric amplification when Josephson junctions are incorporated in them. When Josephson junctions are used, these metamaterials are called Josephson metamaterials which have attracted a good amount of research interest. Because of nonlinear nature of Josephson junction, Josephson metamaterials show nonlinear properties. One dimensional Josephson metamaterials are called Josephson... 

Time-varying electromagnetic structures have provided the possibility of realizing new phenomena and, as a result, new applications, which include non-reciprocity, frequency conversion, amplification, and frequency comb. Among the researches that have been done on time-varying electromagnetic structures, less has been done on the rotating time-varying structure. Rotating time variations have great advantages over other time-varying structures. One of the advantages of rotating time changes is the lack of production of many additional harmonics, which originates from the principle of rotating Doppler shift in frequency. Among its other advantages, it is possible to define a time... 

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.... 

An important and interesting aspect of graphene plasmons (GPs) is their tunability via gating or chemical doping. This property can be used for controlling the propagation of GPs by using spatially non-uniform conductivity profiles in a single flake of graphene. Conductivity patterns are made by locally adjusting the chemical potential of graphene. The chemical potential in turn can be locally tuned by means of chemical doping or gate voltage. Therefore, it is important to know how GPs propagate on a graphene layer with patterned conductivity. In this thesis we will investigate this problem. First, we consider the scattering problem of GPs normally incident on an arbitrary conductivity... 

Metagrating is a new concept for wavefront manipulation that, unlike phase gradient metasurfaces, does not suffer from low efficiency and also has a less complicated fabrication process. However, they have some drawbacks, which we are trying to resolve in this thesis. First, a compound metallic grating is proposed for anomalous reflection. We propose an analytical method for analyzing the electromagnetic response of this grating and show that the structure can have unprecedented near-to-unitary efficiency. Next, two-dimensional (2D) metagratings are proposed for diffraction pattern control. We first present an analytical method for diffraction analysis of 2D compound metallic grating. As a... 

In wave theory, it is conventional to divide media into reciprocal and non-reciprocal types. The Lorentz reciprocity theorem is the most prominent theorem in electromagnetic theory. One wave phenomenon that can be explained by the reciprocity theorem is unidirectional waves. These waves are well-known in the literature and have diverse practical applications. However, in this research, we explore some curiosity-driven questions regarding their behavior near barriers. We hypothesize that new applications might be discovered for these waves. In summary, we analyze the behavior of unidirectional waves when they encounter barriers. The main question is: if unidirectional waves cannot propagate... 

An important issue in High-Frequency circuits is the transmission of electrical signals in Integrated Circuits and Printed Circuit Boards (PCBs). At high frequencies, however, the often used transmission lines suffer from high conductor and substrate loss as well as unwanted electromagnetic coupling to other circuit elements caused by parasitic substrate surface. One method through which both of the above problems may be solved is utilizing waveguide structures on PCBs and ICs. In this way, due to the reduction of conductor loss, a large proportion of signal passes easily. Moreover, and because of closed nature of cavity waveguides, neighboring circuits are protected against electromagnetic... 

Although digital computers have been developed enormously in recent years, analog computing is still of great importance in some specialized application. There are basically two different methods to perform analog computing: metasurface approach (MS) and Green’s function (GF) method. In this contribution, we have focused our attention to the second method. In this approach, the associated Green’s function of the operator of choice is directly performed using a multilayered slab. Despite the applicability of this method, there are two major drawbacks regarding it. The first one is that it is only feasible to perform operators whose Green’s functions have an odd symmetry in the spatial Fourier... 

In this article some systems wich have a resolution beyond the diffraction limit and much more than the conventional lenses (like convex lenses, optical microscopes that have a resolution in order of wavelength) have been reviewed. These structures can be used in many different areas like Electron-Beam lithography (Photoleptography and Nano lithography), printing integrated optical circuits and chips, accurate cell imagings (entering and exiting of proteins in cells) and etc. This kind of imaging is called ”subwavelength imaging”. Generally some remarkable properties of these structures are propagation of all the spatial frequencies or amplifying the evanescent waves. One of the most known... 

By the advance of wireless communication systems, requirements to more advanced systems is increased. Circularly polarized antennas are considered as a robust solution to the demands of contemporary systems because of their unique attributes. This polarization is being used in various systems like satellite communication, global navigation system, mobile communications, wireless sensors, radio frequency identification, wireless power transmission and etc. Different designs are proposed to achieve circular polarization which among them the ones using single feeding techniques are more favored because of their ease of fabrication and excitation. In this thesis, three circularly polarized...