摘要:

Approximate formulas for transient antenna current on an infinite cylindrical antenna caused by a step voltage source, applied at the delta gap, are discussed. A new alternative formula is derived. An asymptotic expansion of the transient solution for late time is given. The Hallén filament approximation, a previous simple formula obtained by averaging, an alternative formula, and the asymptotic expansion are compared to the accurate numerical integration of the exact solution.

ISSN:0920-5071    

DOI:10.1163/156939392X01174

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

A rigorous field analysis of the electromagnetic scattering from a perfectly conducting strip loaded with a dielectric cylinder is presented. The dielectric cylinder is of circular cross-section and the dielectric medium is assumed to be linear, homogeneous, and isotropic. The strip is oriented radially with respect to the center of the cylinder and can be located inside or outside, but touching, the outer boundary of the cylinder. The structure is excited by a plane wave or a line current field and both TM and TE to z excitations are considered. The solution of the scattering from a single loaded strip is based on a boundary value approach and a high frequency asymptotic technique is also developed for the scattering from an array of loaded strips. The validity of this proposed solution is investigated by comparing the numerical results of selected cases with those based on exact and method of moments solutions. Sample numerical results for the scattering from a single loaded strip and an array of loaded strips are presented which illustrate how the far scattered field and the scattering cross-section are affected by the dielectric loading.

ISSN:0920-5071    

DOI:10.1163/156939392X01183

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

A slot fed by a semi-infinite microstrip line can be used as a radiating element or as a circuit element in filter design. This property is investigated using the method of moments in conjunction with the multiple reflection scheme. An open microstrip line section beyond the slot can be used as a tuning stub. To use the length of the stub as a parameter in the analysis, the problem is divided into two canonical parts: 1) an open-ended microstrip line and 2) a slot fed by an infinite microstrip line. Once the necessary reflection and transmission coefficients are calculated for these canonical problems, they can easily be combined, using a multiple reflection scheme, for any length of stub. Moreover, the reflection coefficient due to multiple slots fed in series by the same line can also be calculated, without additional computation effort, from the basic results of the canonical problems, provided that the field coupling between slots is small. This approach is useful in analyzing a slot array fed in series with a microstrip line.

ISSN:0920-5071    

DOI:10.1163/156939392X01192

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Closed-form expressions for uniform high-frequency incremental length diffraction coefficients are derived for perfectly conducting plane strips whose ends are obtained by truncation at the edge of the plane scatterer in question. An essential feature of the derived incremental fields is that the diffraction at the trailing edge of the H -wave, nonuniform leading edge current, which becomes appreciable near grazing incidence, is taken into account even when the leading and trailing edges of the incremental strips are not parallel. These conformal incremental diffraction coeflicients are applied to obtain useful high-frequency solutions to the infinite strip and circular disk for all angles of incidence and scattering.

ISSN:0920-5071    

DOI:10.1163/156939392X01200

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

ISSN:0920-5071    

DOI:10.1163/156939392X01219

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Radio brightness measurements of an incoherent source, in front of a constant-temperature background, depart from the true source brightness because of finite antenna beamwidth, internal receiver noise, and background noise. Noisy measured brightness samples may be used to estimate the measured and the true brightness, at and between the measured points. Such estimates are called "interpolation" and "restoration," respectively. Optimum linear estimation - interpolation or restoration - requires only the second-order brightness statistics of the true and measured brightness samples, and of the corresponding noise samples. We derive these correlation functions in terms of the antenna parameters, subject to the assumptions that the source brightness varies rapidly compared to the antenna beamwidth, and that the background temperature is constant. We apply these results to calculating the interpolation and restoration errors for optimum linear processing of a finite number of noisy measured brightness samples, as functions of the antenna illumination and the signal-to-noise ratio. Analytic results are given for a square array of four measured points. For this case the interpolation and restoration errors increase rapidly as the separation between measured points increases beyond critical sampling, which for an antenna with Gaussian illumination approximates the 3 db half-beamwidth.

ISSN:0920-5071    

DOI:10.1163/156939392X01228

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

This paper addresses the problem of a thin loop antenna of arbitrary radius radiating in an infinitely extending chiral medium. The integral equation for the problem has been formulated and solved using Numerical Electromagnetics Code modified accordingly. It is found that with the additional parameter of chirality one can control the radiation characteristics. It is observed that for a typical case of a loop of radius equal to a half wavelength the radiation resistance increases from about 140 ohms to 310 ohms when the chirality parameter changes from 0.0 (achiral) to 0.003. Also, the patterns become more directive with chirality than in the achiral case for the same electrical size of the loop. The current on the loop decays very quickly with large chirality. Some details of the numerical exercise associated with the moment solution in the chiral NEC code are discussed.

ISSN:0920-5071    

DOI:10.1163/156939392X01237

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

A semi-analytical procedure has been developed to treat scattering of horizontally polarized electromagnetic waves from underground cylindrical inhomogeneities. The air-ground interface is assumed to have a sinusoidal profile. The proposed procedure uses Green's functions theory together with the principle of extended boundary conditions. The unknown electric field inside the scatterer is determined by solving an appropriate integral equation over the cross section of the scatterer. Then the far field in the air is evaluated by employing the steepest descent approximation and the normalized scattering amplitude is plotted for several cases.

ISSN:0920-5071    

DOI:10.1163/156939392X01246

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Analytical expressions are derived for the equivalent magnetic current distributions over the apertures of filled grooves and slits in a thick ground plane. These are obtained via a low frequency solution of the exact integral equations and exhibit the expected edge behavior at the groove or slit terminations. They are used to derive closed-form expressions for the associated characteristic parameters such as aperture potentials, aperture admittances, transfer admittances, radar echo widths, and transmitted powers. Both principal polarizations are treated and the results are potentially useful in constructing suitable models for long and narrow three-dimensional structures.

ISSN:0920-5071    

DOI:10.1163/156939392X01255

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

The diffraction of a plane electromagnetic wave by an open-ended parallel plate waveguide cavity with a planar termination is rigorously analyzed for the E -polarized case using the Wiener-Hopf technique. Introducing the Fourier transform for the unknown scattered field and applying boundary conditions in the transform domain, the problem is formulated in terms of the simultaneous Wiener-Hopf equations, which are solved via a factorization and decomposition procedure. The solution is exact but formal since branch-cut integrals with unknown integrands and an infinite number of unknowns are involved. Approximation procedures based on rigorous asymptotics are further presented and the approximate solution to the Wiener-Hopf equations is derived. The scattered field inside and outside the cavity is evaluated by taking the inverse Fourier transform and applying the saddle point method of integration. Based on these results, numerical computations of the bistatic scattered pattern as well as the radar cross section have been carried out for various physical parameters, and the scattering characteristics of the cavity are discussed in detail. Some comparisons with existing methods are also given and the validity of those methods is discussed.

ISSN:0920-5071    

DOI:10.1163/156939392X01264

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor