摘要:

The input impedance of an inverted-L antenna is derived using the induced EMF method. A sinusoidal distribution of current along the antenna with zero at the end is assumed. Closed-form expressions of the near-fields are derived and used to obtain an expression for the input impedance. The input impedance can be computed using any suitable numerical integration routine. The accuracy of the expression is verified by comparing the results computed using it with that computed using NEC [1]. The formulation presented here can be extended to compute impedances of other antennas like the folded unipole, the loop-loaded monopole and the meander.

ISSN:0920-5071    

DOI:10.1163/156939398X00746

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The evolutionary algorithm for the direct numerical calculation of a nonstationary electromagnetic field in an active (gain or lossy) medium with time varying permittivity and conductivity is presented. This field is described by the time domain two dimension Voterra integral equation of the second kind. The algorithm is based on the time discrete scheme for this equation and does not impose restrictions on the signal shape and duration as well as on time behavior of medium parameters. Parametric amplification of an electromagnetic pulse under harmonic modulation of the permittivity is shown. The influence of dispersion of dissipative medium on electromagnetic pulse transformation under impulse modulation of medium parameters is shown.

ISSN:0920-5071    

DOI:10.1163/156939398X00755

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The problem of a two-dimensional object location and characterization is considered and solved via the concept of an effective model. The nonlinearized 2D inverse problem is solved employing an effective model of a homogeneous circular cylinder. Promising numerical results are obtained for the spatial location, and determination of size and effective constitution of the objects of different topology using poor a priori information and limited measurement data.

ISSN:0920-5071    

DOI:10.1163/156939398X00764

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The inhomogeneously-loaded helix enclosed in a metal cylindrical waveguide operated in the fast-wave regime is field-analyzed using the tape-helix model. The inhomogeneous discrete dielectric supports have been modeled by azimuthally smoothing them out into a number of continuous, homogeneous dielectric tube regions of appropriate effective relative permittivity values determined by the relative volumes occupied by the supports in these regions. While accounting for the radial inhomogeneity, the nonuniformity of the radial propagation constant over the structure cross section is also taken into consideration. Further, structured expressions amenable to easy computation are presented with the help of the relevant recurrence relations, and a method is suggested to use them in an iterative cycle for the desired convergence accuracy. The dispersion relation developed, as a special case, passes on to that obtained elsewhere for a simpler structure in which the dielectric supports for the helix was not considered. It is observed that the dielectric-support parameters become more effective in controlling the dispersion curve than the bare helix parameters. It is also found that the effect of the controlling parameters such as the helix pitch angle on the phase velocity and interaction impedance of the structure becomes less pronounced as the helix is brought closer to the waveguide wall. The presence of the dielectric in the structure increases the value of the interaction impedance to a reasonable value over the entire operating frequency band. The results in terms of the structure parameters, namely, helix pitch angle, helix-support permittivity, helix-to-waveguide wall separation and structure inhomogeneity presented here are rather general, and can be used for the structure design and performance evaluation as well as for the control of any space harmonic of interest in a fast-wave device such as the gyro-traveling-wave amplifier.

ISSN:0920-5071    

DOI:10.1163/156939398X00773

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

In this paper we describe a numerical method for determining the wideband impedance of a transient antenna, based on a method of moments solution of the time-domain electric field integral equation. The impedance is estimated from the antenna's response to a short-pulse transient excitation ; the current is determined directly from the solution of the electric field integral equation and the potential difference between the antenna arms is determined from an integration of the electric near-field. Particular attention is paid to the form of the singular near-field integrals and an alternative representation is given for these in terms of non-singular boundary integrals. The approach described in this paper has several advantages over the previous numerical approach of [1] and is validated against a variety of canonical antennas, including directive and omnidirectional transient radiators. The present numerical method is shown to provide an accurate estimate of the characteristic impedance of such canonical structures (to within 3 % of the known impedance).

ISSN:0920-5071    

DOI:10.1163/156939398X00782

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The microstrip leaky wave antenna (LWA), has been attracting a lot of interest for use in various applications over the past few years due to its low profile, high gain and ease of manufacture. In this paper a spectral domain integral equation formulation is presented which allows for the evaluation of the propagation constant, currents and electric fields excited on the microstrip LWA in all of the three propagation regimes (bound, surface and radiation). Cross-sectional electric field data is presented for both the bound and leaky regimes for the dominant ( EH0), first higher ( EH1) and second higher (EH2) order modes. The results presented in this paper show that there are only minor differences in the field distribution as the operation of the antenna changes between radiation and bound regimes.

ISSN:0920-5071    

DOI:10.1163/156939398X00791

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The objective of this paper is to apply an extrapolation technique in conjunction with the FDTD method to solve scattering problems involving a class of large three-dimensional metallic objects in a numerically efficient manner. The extrapolation technique is based on the observation that, for sufficiently high frequencies, the field variation near the surface of the scatterer exhibits a certain simple frequency dependence and that the knowledge of this behavior can be used to evaluate the induced current at frequencies where the body size is too large to be tractable via direct methods. The solution process begins with an application of the FDTD to obtain the field distribution on a surface enclosing the scatterer over a wide band of frequencies. The computed results in the upper end of the spectrum, where the body size is at least moderately large, are retained and these field solutions are processed by using the generalized pencil of function (GPOF) method to extract the constituent traveling wave components, that usually number only 2 or 3. Finally, the functional frequency dependences of the traveling wave components are extracted and used to derive the extrapolated solutions at higher frequencies. It is demonstrated that the above extrapolation procedure can be applied to a class of three- dimensional scatterers, even at the grazing incidence and in the shadow region, where the asymptotic methods are known to suffer from accuracy problems. An important observation regarding the extrapolation technique is that, in contrast to the direct methods, e.g., the FDTD - and MoM, the extrapolation technique is not constrained, in principle, by any CPU time and memory limitations at high frequencies.

ISSN:0920-5071    

DOI:10.1163/156939398X00809

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

This paper is concerned with the permittivity reconstruction of inhomogeneous dielectric media. The method applies to profiles that vary with depth only, i.e., it provides a one-dimensional profile reconstruction. The data are collected and analyzed in the time domain. In the first part of the paper the theory of the method is reviewed. It is shown that a finite time trace of reflection data suffices to uniquely reconstruct the permittivity profile of the medium. The latter part of the paper presents the experimental set-up and contains also a thorough discussion of the errors that affect the measurements. The inverse scattering algorithm that is used is either based upon an imbedding procedure or on a Green functions approach. The input to either of these algorithms is the reflection kernel or the impulse response of the medium, i.e., the delta function response of the medium. Therefore, a deconvolution of the the measured reflected field and the incident field must be performed. This deconvolution problem is also addressed briefly in this paper.

ISSN:0920-5071    

DOI:10.1163/156939398X00818

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Radiation in an unbounded gyroelectric medium is treated analytically. The electromagnetic field is obtained in terms of the associated dyadic Green's function expressed in the spectral domain as an appropriate superposition of the cylindrical waves. The radiation of an elementary electric dipole is investigated by computing its far field asymptotically to the order (k0r)-1. Simple far field expressions are obtained for dipole orientations both parallel and perpendicular to the biasing DC magnetic field. Very directive radiation patterns are observed in certain cases depending on the orientation of the dipole and the parameters of the anisotropic medium.

ISSN:0920-5071    

DOI:10.1163/156939398X00827

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Thepolarizabilities of an annular aperture in a thick conducting plane are obtained for electric and magnetic cases. The polarizabilities are important parameters to describe the waveguide coupling by apertures. The Hankel transform is applied to express the scattered and transmitted potentials. The problem is solved by a combination of the analytic and numerical methods. The resulting series solution is fast convergent and very efficient for numerical computation. The numerical calculations are performed to illustrate the behavior of polarizabilities in terms of the aperture size and conductor thickness.

ISSN:0920-5071    

DOI:10.1163/156939398X00836

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor