摘要:

By perturbing initial Woodward-Lawson solutions it is possible to design planar array antennas with moderate dynamic range ratios that produce power patterns with arbitrarily shaped beams and low ripple, such as those required of direct broadcast satellite antennas.

ISSN:0920-5071    

DOI:10.1163/156939300X00527

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Electromagnetists use primarily vector algebra developed by Gibbs and Heaviside. However, there are other possibilities and of these probably the most attractive is Clifford algebra. Clifford algebra's elements can be interpreted as geometric entities and its operations as geometric transformations. As a consequence, electromagnetic boundary conditions at an interface can be cast in a geometric form. This form employs reflections with respect to a plane and gives a unified appearance to the boundary conditions in three-space and in space-time. Furthermore, in space-time the geometric form of the conditions is covariant.

ISSN:0920-5071    

DOI:10.1163/156939300X00536

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

A hybrid method of solving Maue's equation for axisymmetric perfect conductors is developed. The "self-patch" improper surface integrals, and those of the adjacent patches, are evaluated analytically by retaining the leading terms of the local Taylor expansions of their integrands. The remaining part of the solution is numerical. The method is applied, as a test, to scattering by spheres and thin dipole antennas and then to disks and cone-spheres of various sizes.

ISSN:0920-5071    

DOI:10.1163/156939300X00545

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The scattering of a plane electromagnetic wave by an infinite dielectric or metallic circular cylinder, parallel with another one, also dielectric or metallic, of electrically small radius, is considered. The method of separation of variables, in conjunction with translational addition theorems for cylindrical wavefunctions, is used. Analytical expressions are obtained for the scattered field and the various scattering cross-sections. Both polarizations are considered for normal incidence. Numerical results are given for metallic and dielectric cylinders.

ISSN:0920-5071    

DOI:10.1163/156939300X00554

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Electromagnetic fields in a rectangular conducting waveguide filled with uniaxial anisotropic media are analyzed in this paper using Fourier transform. TE modes and TM modes are found to be related to the ordinary waves and extraordinary waves, respectively. The calculated dispersion curves are depicted and the effects of the material parameters on the cutoff frequencies are shown. The cutoff frequencies change considerably especially for TM modes. The field distributions are plotted to compare with the isotropic rectangular waveguide. The electric type dyadic Green's function due to electric source is derived in the form of vector wave functions expansion by applying Ohm-Rayleigh method. The final result shows that the parameters of the medium have close and intricate relationships with the properties of the Green's function, hence, influencing the propagation of the guided wave. The dyadic Green's function is reducible to that in the isotropic medium. From Maxwell equations, the magnetic dyadic Green's function due to electric source is also given. As the application of the dyadic Green's functions, numerical results of the fields inside the waveguide excited by an infinitesimal dipole are present.

ISSN:0920-5071    

DOI:10.1163/156939300X00563

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

In this paper the relationship between the range-corrected far fields and the near field measurements is established. We propose methods to obtain far field quantities, including the bistatic RCS, bistatic range profiles, and bistatic images from near field measurements. We show how to obtain the Fourier space data in the Ewald sphere when the object is normally or obliquely illuminated and its scattered fields are received by a probe scanning over a planar aperture. We carry out a simulation of a near-field configuration and calculate near field data. From these data, we derive bistatic range profiles and reconstruct three-dimensional (3-D) bistatic images. We also implement a near-field measurement system and propose a calibration procedure so that the Fourier space data can be accurately obtained. A focused projective image has been reconstructed from near field data measured along one-dimensional scanning. Simulation and experimental results have verified the effectiveness of our algorithms.

ISSN:0920-5071    

DOI:10.1163/156939300X00572

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Scattering from a non-spherical scatterer at microwave frequency has always been an interesting electromagnetic problem. In this paper, the Fresnel zone scattered field of three types of non-spherical discrete scatterers, namely disks, needles and cylinders, are studied. Scatterers of these shapes are commonly used to model the leaves, branches and trunks of vegetation canopy. Conventionally, a far field approximation of the scattered fields from these scatterers is applied in the microwave scattering models to account for the interactions among the scatterers. However, when the scatterers are in the Fresnel zone of one another, it is necessary to consider the additional interaction effect among the scatterers. This effect is considered by including two types of corrections into the far field scattered fields of these scatterers : amplitude correction and Fresnel phase correction. The amplitude correction is considered by including the near field amplitude term of the scattererd fields whereas the Fresnel phase correction is obtained through the consideration of the higher order terms in the phase of the scattered fields. It is found from the theoretical analysis that the effects of these two corrections to the scattering cross section of the scatterers are dependent on the frequency, density, size and orientation of the scatterers. Good agreement is obtained between the theoretical results and the measured backscattering cross sections of rods, disks, birch stick and aspen leaf.

ISSN:0920-5071    

DOI:10.1163/156939300X00581

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

A Hertz dipole radiating in the presence of a dihedral corner constitutes a 3-D wedge-diffraction problem. The first-order physical-optics (PO) solution developed in this paper applies to an obtuse-angled penetrable dihedral corner excited by an electric or magnetic dipole which is either parallel or normal to the edge. A charge distribution is introduced along the apex of the dihedral corner to account for the discontinuity of the PO approximation to the field on the faces at their common edge. Thus, a corrected PO solution is obtained. The integral expression for the field intensity at great distance from the edge is evaluated asymptotically. The resulting formula is uniformly valid and continuous across both shadow boundaries. The effect of the aforementioned charge distribution along the edge is highlighted through a reciprocity check. Maps of every constituent of the field over a coastal corner are presented for all four types of dipole source.

ISSN:0920-5071    

DOI:10.1163/156939300X00590

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Fractional dual solutions to the Maxwell's equations for a TEM plane wave propagating in an arbitrary direction are derived. The results obtained are utilized to calculate fractional dual solutions for spectrum of plane waves radiated by a line source. It is shown that fractional dual field for a plate source and for a line source, in the far-zone along the axis of symmetry, are related via fractional order integral operator. Field that can be regarded as intermediate step between the fractional dual field for a plate source and for a line source is determined. Source distributions corresponding to the fractional dual solutions and intermediate fractional dual solutions are also derived.

ISSN:0920-5071    

DOI:10.1163/156939300X00608

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

This paper presents an analytic formulation of the near- and far-field distributions due to periodic spherical arrays of dipole antennas on two spherical biisotropic substrates, i.e., on the chiral sphere and on a perfectly conducting sphere coated with one-layer chiral medium, respectively. For an arbitrary orientation of the dipole antenna array, the dyadic Green's functions in the form of an eigenfunction expansion in terms of the normalized spherical vector wave functions are directly employed. Thereby, exact representations of the radiated fields in both near and far zones are obtained in closed form. They are all in a superposition of right- and left-handed circularly polarized waves, and are also applicable for the cases of spherical arrays of dipole antennas on spherical isotropic substrates. Furthermore, the constitutive parameter effects of the chiral medium on the three-dimensional patterns of the far-zone field and its polarized feature are demonstrated numerically.

ISSN:0920-5071    

DOI:10.1163/156939300X00617

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor