摘要:

Two dimensional beam diffraction by a 90° -dihedral corner reflector with sides of finite length is analyzed. First the solution for an isotropic electric line source parallel to the corner and edges is obtained for high frequencies by the uniform geometrical theory of diffraction with shadow and reflection boundary positions identified. By introducing appropriate complex coordinates for the source position the solution for a beam directed at the corner is obtained. The shift of shadow and reflection boundaries is plotted for various source directivities. Numerical results for the effect of source directivity on the far field radiation patterns are also shown. The procedure can be extended to high frequency scattering by other finite polygonal structures.

ISSN:0920-5071    

DOI:10.1163/156939398X00115

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

In this paper, Artificial Neural Networks (ANN) are employed for non-linear interference suppression in electromagnetically dense environments. Performance evaluations are concentrated on realistic non-Gaussian interference scenarios simulated by using K-distribution. A simple and effective method is devised to generate the K-distributed interference saunples from the closed form expressions. In addition to the simulated interference, measured values for non-Gaussian harmonic components are added at the input to demonstrate the performance of the canceller in real time environment. A blind alternative to proposed technique is developed in the absence of a training signal and a robust self-orthoganalising interference cancellation system is demonstrated which requires no training sequence. Several simulations are carried out under various channel conditions and using different network architectures to demonstrate that the proposed technique is ideal for applications in electromagnetically crowded hostile propagation environments.

ISSN:0920-5071    

DOI:10.1163/156939398X00124

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We prove that an electromagnetic pulse originating at t = z = 0 and propagating in the z-direction is represented by a Bromwich integral. We discuss the asymptotic approximation of this integral for different saddle point methods. Then, it is shown that pulses have a steady component that propagates with the phase velocity and transient components that propagate with the signal velocity (according to Brillouin's definition). In many cases the signal velocity is the group velocity.We discuss different types of pulses, in particular digital pulses. For practical applications the transients are of the utmost importance since they are the main cause of pulse distortion.

ISSN:0920-5071    

DOI:10.1163/156939398X00133

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

In the recent past, several advances have been made in the development of the next generation personal communication systems. Microstrip antennas with finite ground planes play an important part in these systems. In order to analyze these antennas a new dyadic Green's function for an ungrounded dielectric slab was derived and was used to study a microstrip antenna with the patch and the ground plane of the same size. The dyadic Green's function is used to solve the integral equation derived using the reaction theory. Taking into account the coupling between the patch and the ground plane the current distributions on the patch and the ground plane are determined simultaneously. The radiation patterns of the antenna are determined from these current distributions using the method of moments. A comparison with experimental radiation patterns is presented and the results are in good agreement.

ISSN:0920-5071    

DOI:10.1163/156939398X00142

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A novel numerical technique is presented for electromagnetic scattering analysis for coated flat strips. The computational formula is derived from a coupled system of electric- and magnetic-field integral equations and solved using an adaptive multiscale moment method. In this method, the unknowns are expanded in terms of local-supported triangular functions on different scales which are similar to wavelet-like basis functions. The procedure of solving the problem from a V -scale to a (V + 1) -scale has four steps. The first step is to predict the solutions on (V + 1) -scale from the known solution on a V -scale using interpolation. The second step is to eliminate the smaller coefficients of the basis functions of the solution and omit the corresponding rows and columns of the system matrix obtained from the moment method so as to reduce the size of the linear equations. The third step is to solve the matrix equation by the conjugate gradient (CG) method. The final step is to obtain the solution on the (V + 1) -scale by adding some zero-terms corresponding to the coefficient whose value is zero. Many numerical examples are presented. The results have shown that the novel numerical technique is an efficient, stable and adaptive algorithm.

ISSN:0920-5071    

DOI:10.1163/156939398X00151

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Unlike the scattering problem in homogeneous space, intensive computations of Sommerfeld integrals are involved in the EM scattering of buried scatterers. There are mainly three bottlenecks of the CPU time for such problem: matrix filling, matrix inversion and calculation of the scattered fields. For moderately sized problems, extensive numerical experience shows that the CPU time used in the first and third items (concerned with the Sommerfeld integrals) is much more than that used in the matrix inversion. Therefore an efficient method for solving such buried object problems requires the fast evaluation of such Sommerfeld integrals. In this paper, several efficient methods are presented to evaluate the integrals that appear in the TM wave scattering by two-dimensional buried dielectric and conducting cylinders. In the numerical integration method, the original integrating path is deformed to the steepest-descent paths to expedite the numerical integration and yield a more stable computation result. In the method of steepest descent, the leading and higher-order approximation of the saddle point, and the contribution of the branch point are formulated. However these formulations are invalid near the so called "critical angles". In the uniform asymptotic expansion method, we improve the approximations near such critical angles to yield satisfactory results. Finally, numerous numerical examples are given by using the fast evaluation methods. Numerical results show that the leading-order approximation and the uniform asymptotic solution can expedite the buried object scattering problem, and the CPU time will be reduced several thousands times.

ISSN:0920-5071    

DOI:10.1163/156939398X00160

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

An attempt is made here to integrate and consolidate foundational ideas of electrodynamics and special relativity theories into a consistent structure, from the point of view of the application-oriented applied scientist and engineer. A discussion of this kind is best served by minimizing the sophisticated mathematical tools to the bare minimum, and avoiding lengthy calculations. Some of the difficulties encountered in educating engineers and applied scientist are exactly of this kind, hence a topsy-turvy presentation of special relativity is used, in which fundamental postulates and conclusions exchange roles. With the recent interest in direct time-domain methods, the pre-sent discussion strives to elucidate the fundamental problems involved. Essentially it is argued that appending Maxwell's theory with constitutive relations is in general valid for systems homogeneous in time and space. Spatiotemporal constitutive relations are thus becoming differential operators. Strictly speaking, inhomogeneous systems in time and space are appropriate for non-dispersive systems, or as an approximation. Although Electrodynamics in moving media is often considered a purely academic subject, the understanding of its implementation as proposed by Minkwoski is a crucial cornerstone to our understanding of the physical models at hand. To that end, a generalized approach to the Minkowski method for electrodynamics in moving media is presented. This applies to linear as well as nonlinear media. Finally, a novel representation is proposed for nonlinear constitutive differential operators. This contributes to the spatiotemporal representation of Volterra type nonlinear constitutive parameters.

ISSN:0920-5071    

DOI:10.1163/156939398X00179

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

An electromagnetic scattering model for a layer of randomly distributed vertical long cylinders over a dielectric half space is developed in this paper, which takes into account multiple scattering terms up to second order. Closed-form expressions for the second-order scattering fields are derived for two adjacent cylinders in the near field of each other based on the reciprocity theorem and are validated by comparison with numerical results based on the moment method. By applying the Monte-Carlo simulation technique, the backscattering properties of a layer of conducting cylinders above a conducting ground plane and a layer of dielectric cylinders above a dielectric ground plane are studied. The simulation results show that the HH polarization solution is dominated by single scattering for the layer of cylinders whose diameters are fairly small and lengths are much long compared with the excitation wavelength. The secondary scattering shows great effect on the VV polarization results in the vicinity of the Brewster angle of dielectric cylinders. The simulation results are compared with those obtained by the radiative transfer (RT) method which is based on the addition of intensities and the comparison shows that the RT solutions could not give correct results for the medium composed of vertical long cylinders.

ISSN:0920-5071    

DOI:10.1163/156939398X00188

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We extend the principles of cavity quantum electrodynamics to nuclei inside low-loss cavities. The resulting nucleus-cavity system exhibits spontaneous emission and internal conversion rates which differ radically from those of the individual components. The quantum mechanical derivations of these rates are given. The internal conversion process occurs via the exchange of a virtual photon between an excited nucleus and an electron bound to that nucleus. Even though the photon is virtual, partial suppression of the internal conversion process is shown to be theoretically possible. With a controlled modification of the internal conversion rate using a high-Q cavity, one can achieve a number of potentially interesting applications including an excited nuclear state containment unit, a nuclear battery, and a neutron detector.

ISSN:0920-5071    

DOI:10.1163/156939398X00197

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

In this paper, a statistical electromagnetic (abbr., EM) interference study is constructed mainly from a theoretical viewpoint on the basis of N-dimensional random walk problem in multi-dimensional signal space. First, a characteristic function of the Hankel transform type matched to this EM environmental study is introduced especially in an extended form of D. Middleton's basic result. Then, the probability density function (abbr., pdf) is explicitly derived for the EM random fluctuation with a power scaled variable (or an effective value) measured by a receiver with a mean square operation. The proposed theory is verified by showing the agreement with one of D. Middleton's great researches as a special case of the proposed expressions. Moreover, the effectiveness of the proposed method is experimentally confirmed through an acoustic type simulation of EM environment and the application to a simplified actual EM environment with joint space-time non-Poisson interference as one of the same wave motion type environment as EM environment.

ISSN:0920-5071    

DOI:10.1163/156939398X00205

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor