Currently, more and more practical engineering applications place antenna system on the electrically large platform. This paper deals with the problem of antennas mounted on large platform from two aspects - radiation pattern and system electromagnetic compatibility (EMC). To achieve an accurate and effective computation, this paper applies Method of Moment (MoM) with Higher-order basis functions solver with large-scale parallel computation technique. And finally some real-life examples are presented to describe how to install antennas on the platform reasonably.
We experimentally quantify the radiation of a small Ultra-Wideband (UWB) antenna placed on a human body. The measurements are performed for different antenna locations on the body, namely the head, torso and belt. First the antenna is measured in free space as reference, and then the influences of the body are investigated for different frequencies and antenna polarizations. We observe minimal signal blockage when the antenna is located on the head. It is around 5d B-10 dB loss for all polarizations. For the belt and torso, the blockage is 10 dB to 35 dB, for both polarizations.
The plane-wave synthesis is brought to the CATR. The feed scans in the focal plane and antenna pattern is measured several times in different feed positions. The corrected antenna pattern is obtained by weighting the measured patterns. The weight is obtained by the least squares method for the electric fields in the aperture plane of the test antenna. The fashion, number and spacing of feed scanning are investigated. By the plane-wave synthesis in the CATR, a larger antenna can be tested, and higher test accuracy can be obtained than that of the CATR. Lower test number and less test time are needed than the conventional plane-wave synthesis. The technique proposed in this paper is numerically investigated in an offset single paraboloid CATR and verified experimentally in a Cassegrain CATR at 100 GHz.
A transient quasi-analytic method is improved to analyze the offset reflector for impulse radiating antenna (IRA) applications. Physical optic (PO) approximation and analytic time transform (ATT) are utilized to investigate the time domain (TD) radiating characteristics of the offset reflector. With the appropriate coordinate transformation, the TD far-field integral problem can be simplified to one dimensional angular integral which is independent of the reflector's size. In addition, the Fast Fourier Transform (FFT) of impulse responses is compared to the direct frequency domain result, and good agreement is obtained.
The satellite-borne SAR (Synthetic Aperture Radar) is a quite promising tool for high-resolution geo-surface measurement. Recently, there has been a great interest in Coherent Change Detection (CCD), where the coherence between two SAR images is evaluated and analyzed to detect surface changes. The sample coherence threshold may be used to distinguish between the changed and unchanged regions in the scene. Using COSMO-SkyMed (CSK) images, we show that for changed areas, the coherence is low but not completely lost. This situation, which is caused by the presence of bias in the coherence estimate, considerably degrades the performance of the sample threshold method. To overcome this problem, robust detection in inhomogeneous data must be considered. In this work, we propose the application and improvement of three techniques: Mean Level Detector (MLD), Ordered Statistic (OS) and Censored Mean Level Detector (CMLD), all applied to coherence in order to detect surface changes. The probabilities of detection and false alarm are estimated experimentally using high-resolution CSK images. We show that the proposed method, CMLD with incorporation of guard cells (GC) in the range direction, is robust and allows for nearly 4% higher detection probability in case of low false alarm probability.
In order to study beam-propagation factor (M2-factor) of partially coherent Laguerre-Gaussian (PCLG) beams in non-Kolmogorov turbulence, a generalized exponent and a generalized amplitude factor are introduced. Based on the extended Huygens-Fresnel principle and second-order moments of the Wigner distribution function (WDF), the analytical formula of M2-factor for PCLG beams in non-Kolmogorov turbulence is derived. The corresponding numerical results are also calculated. Results show that for PCLG beams propagating in non-Kolmogorov turbulence, the bigger the beam order or outer scale is, or the smaller the correlation length, C2n, or inner scale is, the smaller the value of the normalized M2-factor is. Furthermore, the normalized M2-factor of PLG beams increases with the increasing of α until it reaches the maximum point, then it gradually decreases with the increasing of α. 2
We present an approach for very quick and accurate approximation of infinite series summation arising in electromagnetic problems. This approach is based on using asymptotic expansions of the arguments and the use of fast convergent series to accelerate the convergence of each term. It has been validated by obtaining very accurate solution for propagation constant for shielded microstrip lines using spectral domain approach (SDA). In the spectral domain analysis of shielded microstrip lines, the elements of the Galerkin matrix are summations of infinite series of product of Bessel functions and Green's function. The infinite summation is accelerated by leading term extraction using asymptotic expansions for the Bessel function and the Green's function, and the summation of the leading terms is carried out using the fast convergent series.
We propose an all optical switch in a dual-core photonic crystal fiber (PCF) that has the core region consisting of soft glass and has nematic liquid crystal filled holes in the cladding region. Light waves are guided in this PCF by total internal reflection (TIR) due to the refractive index contrast between soft glass and liquid crystal (LC). Its wavelength dependent coupling, birefringence and dispersion are calculated and later use these parameters to evaluate the switching characteristics of short pulses propagating through this optical waveguide. The switch demonstrates tunability with external perturbation such as applying external heat source or electric field. Refractive index sensitivity of LC with these perturbation as well as polarization of the light signal determines the coupling, birefringence and dispersion properties of the overall waveguide and its switching characteristics.
Concentric ring antenna arrays with the ability to produce dual pattern have many applications in communications and radar engineering. In this paper, we represent a new method for design of an optimized reconfigurable concentric ring array with dual pattern of desired specifications. Here, our goal is to find a suitable common element excitation amplitude distribution and two different element excitation phase distributions for two desired radiation patterns. For this purpose, we have proposed a novel objective function which is completely different from the traditional objective functions usually used in antenna design problems. For the optimization procedure, we have developed a modified Differential Evolution (DE) algorithm, denoted as DE_rBM_2SX, which employs new kinds of crossover and mutation operators to overcome some drawbacks of the classical DE on single-objective fitness landscapes. We consider three types of dual pattern - pencil beam+pencil beam, pencil beam+flat-top beam, flat-top beam+flat-top beam. The simulation results obtained by applying our proposed method clearly indicate that our method is very convenient to obtain radiation patterns of desired specifications. We compare results of the modified DE algorithm with those of another state-of-the-art improved variant of DE, called JADE and a state-of-the-art variant of the Particle Swarm Optimization (PSO) algorithm called Comprehensive Learning Particle Swarm Optimizer (CLPSO). Such comparisons reflect that the proposed algorithm is more efficient than JADE or CLPSO in finding optimum configuration of the dual pattern concentric ring array antenna. nullS
The effect of graded permittivity profiles, filling factor and incident angles on the dispersion characteristics and reflectivity of binary one dimensional plasma photonic crystals having linearly graded dielectric materials are investigated by using the transfer matrix method. It is observed that position, width of band gap and high reflectance range can be improvised to desired level by proper choice of filling factor and graded permittivity index. The incident angle is found to affect the band gap and high reflectance range. Our analysis also shows that this plasma photonic crystal may be used for sensing applications.
A new structure is suggested for simultaneous microwave chirped pulse generation and array antenna beam steering. It is based on using a multi-channel fiber Bragg grating in a photonic microwave delay-line filter. The paper presents a feasibility study of the idea, discussing the main performance parameters of both signal generation and beam steering functions. Specifically, it focuses on the effects of wavelength tuning, resolution and accuracy. The study shows that custom off-the-shelf components could be used to implement an all optical system capable of generating chirped pulses while steering the radiation pattern of a small sized antenna array. The advantages of the structure for avoiding single sideband modulation difficulties and also for the compensation of the multichannel fiber Bragg grating inaccuracies are also discussed.
The effect of exponentially graded material on the modal dispersion characteristics, group velocity and effective group index as well as phase index of refraction of a binary One-Dimensional Plasma Photonic Crystals (1D-PPCs) has been studied. The dispersion relation is derived by solving Maxwell's equations and using the transfer matrix method. The anomalous dispersion characteristics are observed for different values of selection parameters. The introduction of graded dielectric layers in 1D-PPCs provides additional parameters for controlling the propagation characteristics of 1D-PPCs. Also, the band gap is shown to become larger with the increase of plasma frequency and plasma width. Hence the structure having plasma and exponentially graded dielectric layer in unit cell is more useful for controlling and tuning of the plasma functioning devices than the structure having plasma and homogeneous dielectric layer in one unit cell.
The diffraction problem of a three-dimensional elliptic Gaussian beam on a aperture array of rectangular holes is solved. The both normal and oblique incidences of the beam are considered and the results are presented in the form of the three-dimensional patterns. The pattern lobe distortion and conditions at which the side lobes appear are studied. The conditions under which the shift of the reflected and transmitted field patterns appears are studied. The existence of higher spatial Floquet harmonics in the case of oblique beam incidence is observed.
The objective of this paper is to study the wideband characteristics of the radio channel in a tunnel environment, not only the delay spread, but also the angle of departure/arrival of the rays, their relative weights and their delays, which are important values for Multiple-Input Multiple-Output applications. In order to achieve this goal, a measurement campaign has been carried out in a straight arched tunnel over a frequency band extending from 2.8 to 5.0 GHz and distance varying from 50 m up to 500 m. First, the variations of the channel impulse response and of the delay spread versus the distance between the transmitter and the receiver are analyzed. Then, the bidirectional channel characteristics have been extracted from the measured channel matrices using a high resolution estimation algorithm. The main contribution of this paper is to clearly show the quantitative variation of the delay spread and the angle of departure/arrival of the rays along a real tunnel and to investigate the possibility of using the ray theory in a rectangular tunnel to interpret experimental results obtained in an arched tunnel.
Finding sparsifying transforms is an important prerequisite of compressed sensing (CS) theory. It is directly related to the reconstruction accuracy. In this work, we propose a new dictionary learning (DL) algorithm to improve the accuracy of CS reconstruction. In the proposed algorithm, Least Angle Regression (LARS) algorithm and an approximate SVD method (ASVD) are respectively used in the two stages. In addition, adaptive sparsity constraint is used in the sparse representation stage, to obtain sparser representation coefficient, which further improves the dictionary update stage. With these data-driven adaptive dictionaries as sparsifying transforms for image compressed sensing, results of experiments demonstrate noteworthy outperformance in peak signal-to-noisy ratio (PSNR), compared to CS based on dictionaries learned by K-SVD, in the sampling rate of 0.2-0.5. Besides, visual appearance of reconstruction detail at low sampling rate improves, for reducing of `block' effect.
The present work is motivated by our recent experimental results [2-4] that indicate on anomalously small retardation of bound (or velocity-dependent) electromagnetic (EM) fields in the near zone of an emitter, whereas in the far zone the retardation tends to the standard value determined by the velocity of light c. Such anomaly is specific only for bound field component, while EM radiation has the constant propagation velocity c in the entire space. One possible explanation of these experimental results can be linked to our earlier finding [6, 8] that conventional EM energy-momentum (EMEM) tensor describes bound and radiative EM fields only in spatial regions free of charges and currents. In this work we show that an additional term has to be included into the standard EMEM tensor in order to make viable the description of the whole system of ``charges plus fields". Such approach to the EMEM tensor actually admits anomalously small retardation of bound EM fields in regions very close to a field source, providing the standard propagation in the far zone. Some special implications are also discussed.
It is well-known that, at low frequency, far-field RCS can be measured using a suitable implementation such as outdoor range or large anechoic chamber. The aim of this paper is to propose a new algorithm to predict RCS from near-field measurements. The comparison between RCS values obtained from the proposed method and those obtained from direct far-field values shows a good agreement between the two results.
The study on effects of microwave electromagnetic interference on CMOS RS flip-flops is reported in this paper. Using device simulation method, the relation between the susceptibility of CMOS RS flip-flops and microwave electromagnetic interference frequency as well as pulse width has been analyzed. It is found that the effects of microwave electromagnetic interference get suppressed gradually with increasing frequency. Furthermore, the interference power threshold is inversely proportional to the pulse width, and the interference energy threshold is directly proportional to the pulse width conversely. In addition, because of the difference in the structure of these two categories of CMOS RS flip-flops, they have different susceptibility to microwave electromagnetic interference.
The trapezoidal recursive convolution (TRC) finite-difference time-domain (FDTD) method is extended to study the bistatic scattering radar cross sections (RCS) of conductive targets covered with inhomogeneous, time-varying, magnetized plasma medium. The two-dimensional TRC-FDTD formulations for electromagnetic scattering of magnetized plasma are derived. Time-varying parabolic density profiles of plasma are assumed in this paper. The bistatic radar cross sections are calculated under different conditions using 2-D TE model for a conductive cylinder covered with magnetized plasma. The numerical results show that plasma cloaking system can successfully reduce the bistatic RCS, that the plasma stealth is effective, and that the appropriate parameters of plasma can enhance its effectiveness.
Multiwalled carbon nanotubes display dielectric properties similar to those of graphite, which can be calculated using the well known Drude-Lorentz model. However, most computational softwares lack the capacity to directly incorporate this model into the simulations. We present the finite element modeling of optical propagation through periodic arrays of multiwalled carbon nanotubes. The dielectric function of nanotubes was incorporated into the model by using polynomial curve fitting technique. The computational analysis revealed interesting metamaterial filtering effects displayed by the highly dense square lattice arrays of carbon nanotubes, having lattice constants of the order few hundred nanometers. The curve fitting results for the dielectric function can also be used for simulating other interesting optical applications based on nanotube arrays.