Circularly polarized graphene based transmitarray for terahertz applications is proposed. The characteristics of the graphene material is explained. The cell element of the transmitarray is made of square Quartz cell. Dual circular graphene rings are printed on both sides of the Quartz substrate. The graphene ring radius is varied to change the transmission coefficient phase and magnitude. The effect of the graphene chemical potential on the transmission coefficient is demonstrated. Transmitarray is composed of 9×9 unit cell elements. A circularly polarized circular horn is used to feed the transmitarray at f=6 THz. The left- and right-hand field components in the E- and H-plane are determined. The variation of the gain and the axial-ratio with the frequency are explained. The peak gain is 18.63 dB and 1-dB gain bandwidth is 6.8%. The transmitarray produces a circular polarization from 5.5 THz to 6.5 THz.
Radio refractivity values obtained for different heights (Ground surface, 50 m, 100 m and 150 m) over a tropical station, Akure, South-Western Nigeria using in-situ data over a period of five years has been investigated for chaos. Several chaos quantifiers such as entropy, Lyapunov exponent, recurrence plot were used. Determinism was detected in the time series studied at all the levels. Results obtained from the computation of radio refractivity show that the value of radio refractivity decreases with increasing altitude while chaotic quantifiers obtained at ground level and height 100m are found to be more chaotic than the other two levels (50 m and 150 m).
A novel approach to design microstrip ultra-wideband (UWB) bandpass filter (BPF) using modified genetic algorithm (MGA) is proposed in this paper. To achieve high efficiency and accuracy, conventional GA is modified. By improving the fitness evaluation, selection, crossover, and mutation, the two possible drawbacks of conventional GA, i.e., slow rate of convergence and local-best solution, are overcome. The modified genetic algorithm is then applied to simultaneously search for the appropriate circuit topology and the corresponding electrical parameters with UWB characteristic. To demonstrate the effectiveness of the novel approach, a new microstrip UWB BPF is designed and fabricated. Measurement results agree well with the design index and full-wave EM simulated results.
Excitation of electrostatic ion cyclotron waves (EICW) in a magnetized dusty plasma by an ion beam is studied taking into account the effect of dust particle size, dust particle charge and dust particle number density variations. The presence of dust grain charge fluctuations modifies the dispersion relation for ion cyclotron waves in dusty plasma. It is shown that in the absence of ion beam, the ion cyclotron mode damps due to dust charge fluctuations and an additional damping dust charge fluctuation mode is induced in plasma. The ion beam propagating parallel to the magnetic field drives ion cyclotron waves to instability via Cerenkov interaction. Using the analytical and numerical results the influence of the relative density of negatively charged dust particles on growth rate of ion cyclotron waves is studied. The dust grain size distribution has also significant contributions on the growth rate of ion cyclotron waves.
Based on the Minimum Variance Distortionless Response-Sample Matrix Inversion (MVDR-SMI) method, we propose a novel Adaptive Covariance Estimator (MVDR-ACE) beamformer for adaptation to multiple interference environments. The MVDR-ACE beamformer iteratively determines a minimum number of data samples required while maintaining its average signal-to-interference-noise to be within 3dB from the performance of a theoretical optimum MVDR beamformer and meeting an instantaneous interference cancellation requirement. Finally, based on numerical simulations, we analyze and validate the performance of the MVDR-ACE beamformer. We also compare its performance to the conventional MVDR-SMI beamformer that uses a fixed data sample in its covariance estimator.
An efficient wide-band analysis that combines modified integral equation-physical optics (IE-PO) hybrid formulation with the best uniform approximation is proposed for antennas around an electrically large platform in this paper. The modified single-level Fast Fourier Transform (FFT) algorithm which is based on the subdomain FFT acceleration is employed by interpolating the Green's function and introducing the concept of the empty groups. Furthermore, the correction of the near-interaction is avoided. On the other hand, the best uniform approximation technique is applied to analyze wide-band properties of antennas. Due to the above modifications, the hybrid method needs fewer unknowns and memory requirements than the conventional one.
Mixed near-field and far-field sources localization problem has received significant attention recently in some practical applications, such as speaker localization using microphone arrays and guidance systems, etc. This paper presents a novel space-time matrix method to localize mixed near-field and far-field sources. Using the proposed method, both the direction-of-arrival (DOA) and range of a source can be estimated by the same eigen-pair of a defined spacetime matrix. Therefore, the pairing of the estimated angles and ranges is automatically determined. Compared with the previous work, the presented method offers a number of advantages over other recently proposed algorithms. For example, it can avoid not only parameters matching problem but also aperture loss problem. It has lower computational complexity since the proposed method does not require the high-order statistics or any parameter search. Simulation results show the performance of the proposed algorithm.
Bistatic forward-looking SAR (BF.SAR) has many potential applications, such as self-landing in bad weather and military detection. Therefore, BFSAR receives considerable attention recently. The imaging algorithms for BFSAR are the difficulties of the study. The original Loffeld's Bistatic Formula (LBF) can handle most general bistatic SAR configurations well. But in some complex bistatic geometries, such as high squint or forward-looking cases, the performance of LBF is degenerated. Some extended LBF (ELBF) methods have been developed, which improve the performance of LBF in some special geometries, but still not the forward-looking configuration. In this paper, we modify the LBF method and try to solve the instantaneous azimuth frequencies of transmitter and receiver directly. Then, we can obtain a bistatic point target reference spectrum (BPTRS), which is accurate enough for forward-looking configuration. A range Doppler algorithm (RDA) based on this BPTRS is derived. Finally, simulations validate the accuracy of the modified Loffeld's Bistatic Formula (MLBF) and effectiveness of imaging algorithm.
This paper presents a new approach for localizing mobile phone users using the promising technique of stratospheric platform (SP) flying at altitudes 17-22 km high and a suitable Direction-of-Arrival technique (DOA). The proposed technique provides information about accurate locations for mobile stations - through high resolution DOA technique - which is very important for traffic control and rescue operations at emergency situations. The DOA estimation in this technique defines the user location using MUSIC algorithm which provides good accuracy comparable to the Global Positioning System (GPS) techniques but without the need for GPS receivers. Several scenarios for users' locations determination are tested and examined to define the robustness of the proposed technique.
In this paper, a novel near zero refractive index metamaterial is designed and used as a superstrate of a microstrip antenna. In order to decrease the return loss, particle swarm optimization (PSO) is used to optimize the metamaterial structure. One of the important factors in the antenna designing, which influences the radiation efficiency, is to determine the accurate position of the feed, and PSO is used to find a precise location of the feed with minimum return loss. The simulation and fabrication of the microstrip antenna using the optimized metamaterial structure is also presented. The performance of the antenna is improved, and the gain is increased up to 4.5 dB. The directivity and radiation efficiency are significantly enhanced. Moreover, a very good agreement is observed between simulation and measurement results.
A dual-beam microstrip array antenna based on conical beam elements is proposed in this paper. Circular patch operating at the TM01 mode is used to achieve conical beam pattern. Grating lobes of the array is used to obtain dual-beam patterns with large elevation angle and high gain. Detailed analysis and design have been presented. A 4x4 antenna prototype has been fabricated and measured. Experimental results show that the antenna array has the return loss better than 10 dB over 12.26 GHz-12.88 GHz and exhibits two symmetric radiation beams, directed at ±49.4° with 16.6 dBi gain at 12.5 GHz. Good agreement between the simulated and measured results is observed. Compared with the previous scheme, the alternative proposal possesses the advantages of being easy to form a planar array with low cross-polarization and having relatively high aperture efficiency simultaneously.
The Radar Cross Section(RCS) of moving targets varies dramatically with aspect or time. The accuracy of simulated dynamic RCS is very important for radar system simulation. A novel simulation approach of aircraft's dynamic RCS is proposed in this paper. Firstly, the electromagnetic (EM) model of aircraft is built and the all-space mono-static RCS database calculated. Secondly, the aspect angles (azimuth and elevation) in target coordinate system are calculated from flight path by coordinate transformation. Then dynamic RCS is obtained based on database and aspect angles by linear interpolation method. Account for the influence results from aircraft vibration in target motion, we use a white Gaussian distributed random series to modify the simulated results. The statistical characteristics of three kinds of dynamic RCS values are investigated, and the desirable agreement of results between modification and measurement shows the applicability of this simulation approach.
In this paper, an analytical model of permanent magnet electrodynamic suspension systems (PEDSs) is proposed. Horizontal and vertical magnetic fields of a permanent magnet (PM) are affectively approximated by sinusoidal functions. By this means, closed form solutions are obtained for lift and drag forces of PEDS for the first time. The suspension system is modelled by finite element method (FEM). The analytical values of lift and drag forces are compared with the FEM results. Also, the analytical results are evaluated by experimental results. As so, the accuracy of the analytical model is validated by FEM and experimental measurements.
A novel rotational motion compensation algorithm for high-resolution inverse synthetic aperture radar (ISAR) imaging based on golden section search (GSS) method is presented. This paper focuses on the migration through cross-range resolution cells (MTCRRC) compensation, which requires rotation angle and center as priori information. The method is nonparametric and uses entropy criterion to estimate rotation angle and rotation center, which are used for rotational motion compensation. Experimental results show that the rotational motion in ISAR imaging can be effectively compensated. Moreover, the proposed method is robust and computationally more efficient compared to the parametric methods.
The selection of matching method is critical to the scene matching navigation system, as it determines the accuracy of navigation. A coarse-to-fine matching method, which combines the area-based and feature-based matching method, is presented to meet the requirements of navigation, including the real-time performance, the sub-pixel accuracy and the robustness. In the coarse matching stage, the real-time performance is achieved by a pyramid multi-resolution technique, and the robustness is improved by multi-scale circular template fusion. In the precise matching stage, an improved SIFT method is introduced to calculate the matching position and the rotation angle. To validate the method, some experiments are completed. The results show that the proposed method can achieve the sub-pixel matching accuracy and improve the angle accuracy to 0.1°.
A dual grating waveguide accelerator structure is investigated and compared with the dielectric wakefield accelerator at THz frequencies. In a dielectric wakefield accelerator, thinner liners for a given current and liners having lower dielectric constant are not preferable due to the fact that they generate much lower axial wakefields. This limits the operation of the device at THz. On the other hand, it is shown that a grating waveguide is tuned at THz with shallower slot heights with competitive wakefield gradients than a dielectric wakefield accelerator.
The single-scattering properties of hexagonal columns and plates were studied using Discrete Dipole Approximation at 94GHz, including scattering efficiency, absorption efficiency, asymmetry factor, backscattering cross section and phase function. Random and horizontal orientations of particles were compared, and 35 sizes of maximum dimension D ranging from 1 um to 10 mm were selected. The results indicate that scattering and absorption efficiencies of horizontally oriented hexagonal columns are larger than those of the randomly oriented ones, whereas this phenomenon does not appear to hexagonal plates. The asymmetry factor of horizontally oriented hexagonal plates has a negative value, which means that the backscattered energy is more than forward energy when the particle is large enough. The backscattering cross sections of horizontally oriented hexagonal columns and plates are larger than those of random orientation, which can be explained by that different cross sections of particles will be exposed to incident plane wave. When the particle size is smaller than incident wavelength, little scattering energy difference between random and horizontal orientation exists, while if the particle is larger than incident wavelength, a turning point will happen at θ=110˚, which can be explained by the theory of energy conservation.
In this paper, we develop and present a complete analytical method to analyze the spectral response of a non-uniform multimode fiber Bragg grating assisted devices supporting a few modes. We present the analytical solution while taking into account the two forward and two backward propagating even or odd normal modes of the grating using the matrix method of multimode coupled grating assisted coupler, for sensing application. Earlier, these types of numerical technique based analysis were presented by other researchers, but no one seems to present a complete analytical solution for the given case. The present analytical analysis can simulate a single mode to multimode coupled sensing waveguide devices based on non-uniform grating assisted operation in a coupled structure. The potential applications of our findings will be mostly in sensing devices.
This paper presents a novel technique for the synthesis of unequally spaced linear antenna array. The modified Invasive Weed Optimization (IWO) algorithm is applied to optimize the antenna element positions for suppressing peak side lobe level (PSLL) and for achieving nulls in specified directions. The novelty of the proposed approach is in the application of a constraint-based static penalty function during optimization of the array. The static penalty function is able to put selective pressure on the PSLL, the first null beam width (FNBW) or the accurate null positioning as desired by the application at hand lending a high degree of flexibility to the synthesis process. Various design examples are considered and the obtained results are validated by comparing with the results obtained using Particle Swarm Optimization (PSO), Ant Colony Optimization (ACO) and Cat Swarm Optimization (CSO). Results demonstrate that the proposed method outperforms the previously published methods in terms of a significant reduction in peak side lobe level while maintaining strong nulls in desired directions. The flexibility and ease of implementation of the modified IWO algorithm in handling the constraints using static penalty function is evident from this analysis, showing the usefulness of the constraint based method in electromagnetic optimization problems.
We numerically investigate the optical spectra of a photonic band gap material realized by one-dimensional Triadic-Cantor quasi-periodic structure. The studied system is composed of two elementary layers H and L with refractive indices nL=1,45 (SiO2) and nH=2,3 (TiO2), respectively. Analytical calculations using a trace and antitrace maps approach have been used to find the reflection and transmission theoretical expressions in visible range under quarter wavelength condition. In our results we present the effect of iteration order of Triadic-Cantor sequence on the optical properties of these multilayer systems, namely the photonic band gap behavior and the optical windows presence, which makes this type of structures good candidates for interesting applications in the field of the nano-optical engineering.
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