摘要:

A vertical monopole on shipboard for communication in the very broad band from 1.5 MHz to 30 MHz is analyzed approximately taking into account its location on the upper deck. The current distribution, impedance, far-field pattern, and efficiency of the resistively loaded monopole are determined and compared with the same monopole located directly on the surface of the sea. It is shown that with an input power of the order of 5 kW, an adequate field is maintained near the surface of the sea over the entire frequency band with a reasonably constant driving-point admittance.

ISSN:0920-5071    

DOI:10.1163/156939395X00226

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Thispaper presents a study of the Fuzzy ARTMAP neural network in designing cascaded gratings and frequency selective surfaces (FSS) in general. Conventionally, trial and error procedures are used until an FSS matches the design criteria. One way of avoiding this laborious process is to use neural networks (NNs). A neural network can be trained to predict the dimensions of the elements comprising the FSS structure, their distance of separation, and their shape required to produce the desired frequency response. In the past, the multi-layer perception architecture trained with the back-prop learning algorithm (back-prop network) was used to solve this problem. Unfortunately, the back-prop network experiences, at times, convergence problems and these problems become amplified as the size of the training set increases. In this work, the Fuzzy ARTMAP neural network is used to address the FSS design problem. The Fuzzy ARTMAP neural network converges much faster than the back-prop network, and most importantly its convergence to a solution is guaranteed. Several results (frequency responses) from cascaded gratings corresponding to various angles of wave incidence, layer separation, width strips, and interstrip separation are presented and discussed.

ISSN:0920-5071    

DOI:10.1163/156939395X00235

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A method for modeling small aperture scattering in the Finite-Difference Time-Domain (FDTD) method is described and evaluated. The method replaces a small aperture in a perfectly conducting screen with pairs of oppositely directed electric and magnetic dipoles on either side of the short-circuited screen. Equations are derived which determine the electric and magnetic currents induced by arbitrary fields within the FDTD computational domain. These currents are induced by the short-circuit fields at the aperture. To determine the short-circuit fields it is necessary to subtract out the fields of the electric and magnetic dipoles. An analytical solution for dipole radiation is used to determine the dipole fields, and it is shown that the FDTD dipole fields deviate from the continuum dipole fields at distances close to the dipole. The method is evaluated for an isolated circular aperture and the results are compared with other methods. An analysis of the various errors inherent in the method is given. The analytical solution to the FDTD equations is used to determine the extent of discretization error inherent in FDTD. It is shown that a very accurate solution for the power transmitted through small circular aperture can be obtained using the proposed method.

ISSN:0920-5071    

DOI:10.1163/156939395X00244

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A technique for computing the electromagnetic (EM) radiation and scattering from three-dimensional (3D) conducting bodies of general shape is presented. The arbitrary surface is described by dividing it into a number of connected patches which are mathematically described as parametric quadratic surfaces. The electric field integral equation (EFIE) is discretized by the standard method of moments (MOM) with specially designed basis functions for subdomains which now contain surface curvature. The use of the basis functions on curvilinear patch reduces the number of unknowns used in MOM.

ISSN:0920-5071    

DOI:10.1163/156939395X00253

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Duality transformations are defined as linear transformations between pairs of vector fields and sources. A class of bianisotropic media is defined so that a pair of duality transformations can be found which leave the media invariant (self dual). It is seen that for such media the permittivity, permeability and nonreciprocity parameters must be multiples of the same dyadic while the chirality parameter may be an arbitrary dyadic. Any source and field in such a medium can be decomposed in a pair of self-dual components which see the bianisotropic medium as two equivalent anisotropic media of affine-isotropic type. The Green dyadics in these two media can be solved through affine transformations and the Green dyadics of the original bianisotropic medium can be written in explicit analytic form. The plane-wave dispersion equation can be solved through a similar self-dual decomposition of fields.

ISSN:0920-5071    

DOI:10.1163/156939395X00262

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Sommerfeld-type integrals are frequently encountered when deriving the fields due to current elements radiating over a lossless or lossy grounded slab. In this paper, the problem of evaluating the electromagnetic field sustained by an electric source embedded in stratified struetures with planar boundaries is completely solved in the spectral domain. The Sommerfeld integral representation of the microstrip Green's function converges rapidly for source and field points which are vertically separated with respect to the interface plane and becomes very poorly convergent when the source and the observation points are radially or laterally separated. For the case of source and the observation points vertically separated, we improve the Chebyshev decomposition method by evaluating the Chebyshev coefficients via the Fast Fourier Transform. As the lateral separation among source and observation points increases the usual Sommerfeld integrand starts to blow up and to oscillate rapidly. In order to evaluate the otherwise divergent and highly oscillatory Sommerfeld integrals, two different deformations from the real axis to the complex plane of the original Sommerfeld contour of integration are presented. The first one is a deformation over the lower-half of the complex plane, taking account for residues and branch cuts. The second one is an integration over the first quadrant of the complex plane. An extensive discussion of the saddle point methods of integration necessary for approximated closed form representation of the electromagnetic field together with applications of the theory to the explicit determination of the electromagnetic field radiated by sources in isotropically stratified planar regions with piecewise constant properties along y are, then, presented. Finally, numerical evaluations of the modulus of the electric field, sustained by an horizontal electric point-source over the planar interface and a comparison between the integration of the radial propagation representation and the closed asymptotic expansion are shown.

ISSN:0920-5071    

DOI:10.1163/156939395X00271

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A corrugated circular waveguide is proposed as a microwave filter. The analysis is carried out using the perturbation method of multiple scales for the case of TM modes. The analysis concerns the interaction of two and four modes satisfying the resonant condition (Bragg condition) imposed by the periodicity of the waveguide wall. The coupled mode equations derived via the method of multiple scales are used to formulate the filtering problem as a two-point boundary-value problem which is solved numerically. Desirable filtering characteristics may be realised by introducing tapered as well as chirped corrugations to control the frequency response of such a wave filter. In case of two-mode interaction the side ripples can be eliminated by tapering the waveguide wall. In the four-mode interaction case, a multichannel filter may be realised by combining taper and chirp.

ISSN:0920-5071    

DOI:10.1163/156939395X00280

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Ridged waveguide is a classic arrangement utilized in broadband microwave components. An analysis of triply-ridged waveguide, presented here, indicates that the dominant mode consists of a degenerate pair. Degenerate mode pairs are widely used in phase shift and control applications. It also exhibits single mode bandwidth characteristics more favorable than the conventional quadrupally-ridged cross-section. Modal hierarchy and field patterns are provided through the use of a finite element solver. Experimental results obtained from a triply-ridged cavity are also discussed and compare well with the numerical calculations.

ISSN:0920-5071    

DOI:10.1163/156939395X00299

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

An exact formulation of the dyadic Green's function in terms of new spherical vector wave functions for the electromagnetic sources placed inside of a radially multilayered sphere of reciprocal and nonreciprocal chiral media is presented according to the method of scattering superposition, and the general formulas for the determining the focal points of spherical chirolens are derived using the approximation of impedance matching and geometrical optics. Furthermore, we investigate the electromagnetic responses of inhomogeneous spherical chirolens antenna characterized by different constitutive parameters, and the effects of impedance matching as well as chirality admittance on the radiation patterns are examined in detail.

ISSN:0920-5071    

DOI:10.1163/156939395X00307

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A domain decomposition method is introduced to facilitate the efficient and rigorous computation of electromagnetic phenomena in structures that are electrically large in one dimension. These large structures are decomposed into many smaller regions by placing partitions throughout the structure, then numerical solutions are generated within each region. Field continuity conditions are applied at the partitions between regions after the numerical solutions are generated to form the solution of the entire structure. By partitioning the large structure into smaller independent regions, the boundary value problem is rendered solvable on a workstation environment, but is also made suitable for massively-parallel computation. Finite element techniques, used for the numerical solutions in this paper, have several levels of parallelism that can be taken advantage of in a parallel environment. Repetitious or nearly periodic structures can be analyzed much more efficiently, since the numerical solutions in unique regions need only be generated once, rather than every time they occur in the total structure. Examples demonstrate the accuracy of the method in a 'finite' periodic structure, and explore the trends in computation time on the Connection Machine (CM-2).

ISSN:0920-5071    

DOI:10.1163/156939395X00316

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor