Asymptotic and Hybrid Methods in Electromagnetics
Asymptotic methods provide considerable physical insight and understanding of diffraction mechanisms and are very useful in the design of electromagnetic devices such as radar targets and antennas. However, difficulties can arise when trying to solve problems using multipole and asymoptotic methods together, such as in radar cross section objects.
Inspec keywords: electromagnetic wave diffraction
Other keywords: asymptotic current; asymptotic theory; electromagnetic creeping wave; hybrid diffraction coefficient
Subjects: Electromagnetic wave propagation; Electromagnetic waves: theory
- Book DOI: 10.1049/PBEW051E
- Chapter DOI: 10.1049/PBEW051E
- ISBN: 9780863414473
- e-ISBN: 9781849190404
- Page count: 262
- Format: PDF
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Front Matter
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1 Asymptotic theory of diffraction
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This chapter discusses the asymptotic theory of diffraction. The geometrical theory of diffraction (GTD) was conceived by Keller. Later, he showed that diffraction phenomena can be incorporated into a geometrical strategy and phrased in geometrical terms by introducing diffracted rays. These rays have paths determined by a generalisation of Fermat's principle. The concept of diffracted rays was developed by Keller from the asymptotic evaluation (as the wave number k tends to infinity) of the known exact solution to scattering from simple shapes, referred to as the canonical problems of GTD.
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2 Electromagnetic creeping waves
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In tins section, we turn our attention to the boundary-layer problem associated with creeping waves on a general surface. Creeping waves can be launched by a distant incident wave or by a source located on the surface of the object.
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3 Hybrid diffraction coefficients
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This chapter discusses hybrid diffraction coefficients. A hybrid diffraction coefficient is a combination of the diffraction coefficients corresponding to two types of diffraction phenomena. We consider here the combination of edge diffraction with the launching of creeping waves or the reciprocal situation of a creeping wave diffracted by an edge.
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4 Asymptotic currents
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This chapter discusses asymptotic currents. The method of asymptotic currents is a natural extension of physical optics (PO) and the physical theory of diffraction (PTD), which are founded on the radiation of the geometrical optics (GO) currents on the illuminated part of a target augmented by the radiation of the fringe currents close to the edges. Compared to PO, the asymptotic current method takes into account the currents on the shadow side of the target by introducing the creeping waves on convex surfaces and the whispering gallery modes on concave surfaces.
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5 Hybrid methods
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Hybrid methods were introduced in the early 1970s for solving some difficulties encountered with asymptotic methods. A typical situation is the diffraction of an object which is large compared to the wavelength but which has locally in some specific areas a complexity in the geometry (fine details) or in the boundary conditions (thick penetrable material) which fall outside the domain of applicability of asymptotic methods. To this category of targets, we also need to add singularities of the surface for which no explicit expressions for the coefficient of diffraction exist.
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Back Matter
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