Sharif Digital Repository

We propose a new inverse scattering technique based on sparsity for the application of microwave imaging. The underdetermined inverse problem appeared in the distorted born iterative method (DBIM) technique is solved using the suggested L2-regularized iterative weighted algorithm (L2-IWA). The L2-regularizer has been introduced to stabilize the algorithm against nonlinear approximations, and the sparsity is enforced with the aid of another reweighted L2-norm regularizer to address the ill-posedness of the inverse problem. The derived algorithm is a three-step iterative technique which solves the underdetermined set of equations at each DBIM iteration. Moreover, the convergence of the L2-IWA... 

in this paper the phase distribution in an array of coupled oscillators arranged on a circular surface is discussed. Coupling is considered to be weak and each oscillator is coupled to its four nearest neighbours. In a continuum limit where the number of oscillators goes to infinity a theoretical formulation in polar coordinates is proposed to describe the dynamic behaviour of phases of the array elements. A Green's function for phase distribution of the array is proposed in terms of its normalized eigenfunctions. Beam steering can be done via detuning the edge oscillators. © 2009 EuMA  

Dracocephalum kotschyi essential oils obtained by hydrodistillation (HD), microwave-assisted hydrodistillation (MAHD), and solvent-free microwave extraction (SFME) were investigated by GC-FID (Gas Chromatography-Flame Ionization Detector) and GC-MS (Gas Chromatography-Mass Spectrometry). The percentage of oxygenated compounds was significantly increased from 62.52% in HD to 76.47% in MAHD, and 84.52% in SFME. Conversely, the monoterpene hydrocarbons were decreased from 30.84% in HD to 13.71% in MAHD, and 5.85% in SFME. The main compound in the essential oil obtained by HD is limonene, which accounted for more than 30% of the oil, while the percentage of this compound was reduced to 9.52% in... 

ZnO–CeO2 nanoparticles were synthesized by microwave combustion and deposited by electrophoretic deposition (EPD). The effect of using two different alcohols, ethanol and methanol, was investigated on EPD behavior and morphology of deposited film. Moreover, the effect of concentration of nanoparticles and applied voltage on the mass of deposit and the variation in the current density were investigated. With a change in the alcohol type, the surface morphology of deposition changed and some voids were observed on the deposition surface in ethanol. In all cases, with increasing concentration of nanoparticles in suspension, the number of developed cracks increased. Besides, a rise in voltage... 

Nonlinear transmission lines (NLTLs) are one of the most prominent solid-state sources of high-power RF pulses. However, there is no integrated mathematical basis, which predicts adequately the generated waveforms on an NLTL and those delivered to the load. This analysis is required for designing and optimizing these transmission lines. Most works in this area are related to experimental studies and results. In this paper, we have developed a theoretical basis, which predicts all the required characteristics of the generated RF pulses on a lossy NLTL. The RF amplitude, the maximum voltage, and the central frequency of the generated pulses at each node of an NLTL are formulated. All the... 

A new resonance phenomenon is demonstrated in waveguide cavities, which simultaneously uses two orthogonal modes (polarizations). This resonance is formed by bouncing waves with similar handedness, between two simple anisotropic metasurfaces having a relative rotation angle. The rotated anisotropic metasurfaces can cross couple the waves from one polarization to the other at the cavity end. The field profile of the resonant mode does not exhibit nodes and antinodes, thus the resonant frequency is not solely determined by the cavity length, unlike common resonators. The resonance condition is theoretically demonstrated from both field and transmission line perspectives, and is validated by... 

The technique of adaptive spatial resolution is for the first time applied in fast and efficient Fourier-based analysis of metallic lamellar gratings at microwave frequencies. Inasmuch as the ultrahigh-contrast permittivity profile of these structures is likely to incur numerical instabilities, the continuity condition is heedfully imposed on the transverse electromagnetic fields and an elegant, unconditionally stable matrix-based strategy is proposed to rigorously analyze the microwave transmission of these structures. © 2009 IEEE  

Monolayer band-reject frequency selective surfaces (FSS) based on curved coupled microstrip line (CCML) resonators are proposed. Each unit cell comprises a single- or multi-loop CCML resonator, featuring single-wideband or multi-band resonance, respectively. Design guidelines for achieving various responses are demonstrated and it is shown there is a great flexibility attaining desired frequency responses versus structural parameters. For instance, it is shown that the entire microwave X-band may be rejected, using a simple single-loop structure. The FSS is particularly useful for protecting buildings against parasitic microwave signals. A visual transparency factor for the FSS, when regular... 

In conventional high-voltage power supplies used for driving the microwave tubes, the shunt crowbar, which generally consists of series-connected thyristors, protects the microwave tube against the vacuum arc fault. In the case of arc fault, the thyristor-based crowbar is closed as a parallel path and diverts the fault current through itself. It avoids serious damage to the microwave tube. However, thyristor switches usually have a limited rate of rise of ON-state current (di/dt), which necessitates placing a series inductor in the crowbar path. Thus, satisfying all protection limits of microwave tubes-including the injected fault energy and the fault current interruption time-is a... 

In this work, the authors experimentally show Dirac Leaky Wave Antennas (DLWAs) at upper microwave frequencies. For the first time, DLWAs are implemented using simple Parallel plate waveguide (PPW) technology, while yielding desirable radiation features and continuous beam scanning through broadside, as well as extremely low profile, with significant ease of fabrication, making them well suited for Ku band applications such as satellite communication, radar and emerging fifth-generation (5G). A planar Dirac photonic crystal in PPW is shown with a closed bandgap and linear dispersion around broadside. In this work, 1D and 2D PPDLWAs are designed that provide scannable fan and pencil beams,... 

Currently, materials with outstanding absorption abilities, such as thin size, better absorbing power, and light weight are the need of industry to resolve the electromagnetic issues. However, the research on optimizing the composition of the material, microstructure and the structure of the absorber are also the important factors for enhancing the absorption features. A metamaterial microwave absorber (MMA) based on nano ferrites with desirable absorption peaks is proposed and simulated. Sol-gel auto combustion route is used to prepare the nanosized Sm doped Co ferrite with Co1+xSmxFe2-2xO4 at x = 0.00, 0.03, 0.06, 0.09, respectively. XRD, VSM, FESEM, and VNA were employed to evaluate the... 

Tunable metamaterials functionalities change in response to external stimuli. Mechanical deformation is known to be an efficient approach to tune the electromagnetic response of a deformable metamaterial. However, in the case of large mechanical deformations, which are usually required to fully exploit the potential of the tunable metamaterials, the linear elastic mechanical analysis is no longer suitable. Nevertheless, nonlinear mechanical analysis is missing in the studies of mechanically tunable metamaterials. In this paper, we study the importance of considering nonlinearity in mechanical behavior when analyzing the response of a deformable metamaterial and its effects on electromagnetic... 

Reaction of aromatic α-bromoketones, sodium azide and aromatic or aliphatic terminal acetylenes in the presence of Cu/Cu(OTf)2following the classical method (aqueous acetonitrile at room temperature) and under microwave irradiation (H2O at 85 °C) leads to 1,4-disubstituted 1,2,3-triazoles as the final products after simple filtration  

We review our investigations on bismuth-oxide fiber as an amplifying medium for parametric amplifiers with continuous-wave pumping. The results lead to new applications in demultiplexing, which are discussed  

Ideally the processing gain in passive radar is PG=BT. Where B is the bandwidth and T is the integration time. So by increasing T the target detectability will also increase. But practically, when we deal with moving targets, increasing T will result in a problem: target migration from its range cell that in turn results in decreased processing gain. In this paper we have suggested a method, by which we can compensate target's migration, while keeping the computational load low  

In this paper, we develop a phase inverting technique to improve the received power of an antenna array, with an emphasis on the simplicity of the receiver hardware and reducing the power consumption. The goal is to propose a simple array architecture to develop intelligent algorithms to enhance the received power of the array. A test set-up with 8-element switched array antenna at 28 GHz has been developed to implement the proposed algorithms. The measured results reveal that in a high scattering environment, the phase inverting technique provides about 13 dB enhancement in the received power compared to a fixed array. This significant enhancement plays a major role in establishing a robust... 

Feed-forward is one of the main methods for linearization of microwave power amplifiers. Although the complexity of the system is increased, the level of inter-modulation rejection (IMR) of a feed-forward system can be improved using a multi-loop configuration. Predicting the IMR of such a system is of prime importance. A new analytical approach in modeling a multi-loop feed-forward system for predicting the linearization performance of the system is presented. The first loop of the multi-loop system is substituted by an equivalent amplifier model and then the reduced system as a new feed-forward system is evaluated. The predicted IMR values are verified using the ADS simulator and a MATLAB...