Sharif Digital Repository

Phase noise is one factor determining the quality of radar and telecommunication systems. Oscillators play an important role in system phase noise, as a signal generator for information modulation. Due to the greater impact of flicker noise at near carrier frequency than other types of phase noise sources, its reduction will greatly impact the phase noise of the telecommunication system. In this research, the simulation of phase noise and flicker noise of an oscillator with various types of bias circuit methods, such as bias with two independent sources, self-bias, improved self-bias, active bias, and low-frequency feedback, has been done. Then, a microwave dielectric resonator oscillator... 

It is well known that the fluctuations of electromagnetic fields in vacuum or in material media depend on the boundary conditions imposed on the fields. This dependence gives rise to forces which are known as Casimir forces, acting on the boundaries. Casimir forces between similar and disjoint objects such as two conducting or dielectric bodies are known in most cases to be attractive.¬¬ These forces are sometimes viewed as the macroscopic consequence of Van der Waals and Casimir-Polder attraction between molecules. In this thesis at first, calculation of Casimir forces in simple models were demonstrated. Then Van der Waals and Casimir-Polder forces have been explained in a unified theory... 

In this thesis numerical investigation on the effect of injecting plasma into the MILD combustion regime which is based on the low oxygen dilution and high preheated temperature, were carried out. The burner which was utilized in this study is a co-flow burner, consists of three coaxial channels. From the center to the outside, there is a fuel jet at the center part, a co-flowing dielectric barrier discharge (DBD), and a preheated air section that is vitiated. The DBD that was used in this study is generated between two coaxial electrodes of which the outer one (conical copper electrode) is covered with a dielectric material made of quartz. The burner is modeled in a 2D-axisymmetric... 

Volatile organic compounds (VOCs) has harmful environmental and health effects and appropriate processes are needed to remove them from industrial and indoor environments. In this study, formaldehyde as one of the most toxic VOCs was investigated. Various methods have been developed to remove formaldehyde, however due to the to their low removal efficiencies, secondary pollution, and low energy efficiency, development of alternative methods will be beneficial. In recent years, the use of a combination of non-thermal plasma and catalyst technologies, called plasma-catalytic hybrid reactors, has provided significant results in the fields of chemical synthesis and removal of pollutants. In the... 

In this study, an Experimental investigation of plasma-assisted combustion using a DBD actuator has been performed. This actuator ionizes the fluid flow by producing asymmetric and cold plasma. Ionizing the flow of fuel or air in the combustion chamber reduces the combustion time scale. The test chamber consists of two coaxial quartz tubes that fuel passes through the central tube and the air pass through the side tube. In the case of fuel (natural gas) ionization, one rod as the electrode passes through the center tube, and the second electrode closes in a loop around the inner tube. One annular electrode is closed around the central tube during air ionization, and the second electrode is... 

In the current study, first, the presence history of dielectric barrier discharge (DBD) plasma actuator in flow control usages has been investigated. After recognizing the importance of plasma actuator in this branch of engineering, this active controller was modeled relying on computational fluid dynamics (CFD) knowledge to control flow separation. In order to model this actuator, physics based model called spilt-potential model and modifications done to improve it, were used. Using this model and solving two elliptic equations, electrical field and plasma charge density distribution are obtained in range of solution domain and body force from plasma actuator modeling in all computational... 

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... 

Microwave and millimeter wave engineering are now crucial to modern life. This field has developed as a result of its applications in non-destructive testing, communications, radars, imaging, and medicine. Oscillators generate the reference frequency in all of these applications, therefore utilizing an appropriate oscillator may have a significant influence on the performance and even the additional expenses of a microwave system. Phase noise is an example of an oscillator’s characteristic that is especially crucial. This characteristic results in a number of limitations in the system’s final performance. For instance, the phase noise of the local oscillator will be one of the causes of... 

Breast cancer is one of the most common cancer types and is the leading cause of cancerrelated death among females according to American Cancer Society statistics. Breast screening and early detection is the most effective method of management and treatment of this disease. Currently, several imaging systems for breast cancer detection are being used among which microwave imaging has caught the attention of many as a promising tool for early breast cancer detection. Microwave imaging is considered as a non-invasive, relatively low-cost, Fast, convenient, and safe method for breast cancer screening. this research aims to implement a 3D imaging system that operates based on the microwave... 

Today, microfluidics, representing the precise and controlled displacement of small amounts of fluid, is one of the most efficient tools available in various research fields, including medicine. Digital microfluidics is one of the newest microfluidic methods, which is based on the theory of electrowetting on dielectric. According to this theory, by applying a voltage difference to a droplet of fluid on a hydrophobic surface, the droplet can be moved on that surface. Therefore, by fabricating a plate containing a number of electrodes completely isolated from each other and controlling them, small droplets of fluid can be moved on a hydrophobic surface. In this thesis, the electrodes of this... 

The goal of the research presented in this thesis is assessment of millimeter-wave (mm-wave) reflectometry as a potential method for non-invasive detection of skin cancers. The reasons for choosing mm-waves are lower penetration, higher resolution and technology maturity. The main work of this research is numerical simulation of reflection from skin and other media as well as reflection measurement of some skin-equivalent phantoms at mm-wave frequencies. By exposing the skin to mm-waves and studying the characteristics of the reflected wave malignant lesions are differentiated from benign and normal skin tissues. Since Oxygen molecules have strong attenuation peaks around 60 GHz band, this... 

Here we have used CFD to simulate the flow field around a DBD plasma active controller for flow control. Enhanced Electrostatic model is applied to model the plasma actuator, by solution of two elliptic equations to find electric field and charge density in whole flow field. So, it provided the body force by neglecting the magnetic forces in Lorentz equation. The body force is added to the momentum equation as a source term. A commercial software FLUENT is used for this simulation. To validate the algorithm, flow over a flat plate using DBD actuator is solved and results are compared with experimental and numerical results. Flow control around a cylinder with Reynolds number of 18,000 is... 

In this study, the substituted barium hexaferrite nanoparticles with general formula Ba1-xCexFe12-xCoxO19 (x = 0, 0.1, 0.2 and 0.3) were prepared by the sol-gel auto combustion method. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FE-SEM), Mossbauer spectroscopy (MS), vibrating sample magnetometer (VSM) and performing network analyzer (PNA) were employed to investigation of structural, magnetic, dielectric and microwave properties of barium hexaferrite nanoparticles. Moreover, differential scanning calorimetry (DSC) were performed to observe the formation mechanism of barium hexaferrite. DSC studies showed that barium... 

Today, hydrogels have various biomedical applications as three-dimensional networks with the ability to absorb and retain water and biological fluids. This study aimed to prepare nanocomposite hydrogels from polyvinyl pyrrolidone, polyethylene glycol, and barium titanate nanoparticles. Gamma radiation at irradiation doses of 25 and 35 kGy was used as the hydrogel's crosslinking agent. The effects of gamma irradiation dose, percentage of polyethylene glycol, and barium titanate composition on the swelling ratio and gel content were investigated. It was observed that the amount of gel content with the addition of polyethylene glycol and barium titanate nanoparticles reduces the gel content and... 

Production of nano-particles using arc-discharge method in a liquid nitrogen environment is a proficient way of forming nitride Particles whose formation by other methods is difficult and expensive. Amongstall, Titanium nitride (TiN) is of significant importance because of its high formidability and its optical and electrical metal properties due to plasmonic electrons. This research studies the production of titanium nitride Nano-particles using arc-discharge method and investigates optical properties and features, specially dielectric constant and optical extinction coefficient. Modeling interband Transitions is also discussed to revise Drude's electron theory. First, in experimental... 

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... 

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... 

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... 

Finite dielctric structures are commonly used in optical devices, Microwave Integrated Circuitc (MICs) and printed antennas. To analyze these structures, full-wave analysis methods cannot be employed easily. They need huge computer resources and are time-consuming especially for electrically large structures. On the other hand, asymptotic techniques may not be exact enough for these structures. In this thesis, MPIE technique is chosen as an accurate and efficient technique for analyzing these structres. To use this technique, the magnetic vector potential and electric scalar potential are required. Uniform and closed-form spatial Green's function for finite dielectric structures is... 

In recent years, there are continuous researches on replacing new dielectrics with suitable dielectric properties (including high dielectric coefficient, low loss factor, high breakdown voltage, and high nonlinear coefficient simultaneously). One of these new materials is co-doped titanium oxide with donor and acceptor of electron, which in spite of having high dielectric coefficient and low loss factor. it has low breakdown voltage and researches on improving the disadvantages of this dielectric are ongoing. Due to the structural similarities between tin oxide and titanium oxide, the tin oxide can be an interesting system for studying and improving disadvantages in this field. In this study...