Propagation, Scattering and Dissipation of Electromagnetic Waves
Describes highly effective, rigorous analysis methods for electromagnetic wave problems.
Inspec keywords: anisotropic media; resonators; surface impedance; electromagnetic wave diffraction
Other keywords: anisotropic media; dissipation process; resonator; corrugated waveguide; eigenmodes; electromagnetic wave diffraction; finite conducting wall; surface impedance technique
Subjects: Waveguide and microwave transmission line components; Electromagnetic wave propagation; Electromagnetic waves: theory; Waveguide and cavity theory; Superconductor response to electromagnetic fields
- Book DOI: 10.1049/PBEW036E
- Chapter DOI: 10.1049/PBEW036E
- ISBN: 9780863412837
- e-ISBN: 9781849193924
- Page count: 288
- Format: PDF
-
Front Matter
- + Show details - Hide details
-
p.
(1)
-
1 Introduction
- + Show details - Hide details
-
p.
1
–17
(17)
An important feature of microwave waveguides and resonators is heat losses. The latter determine, in particular, the transient time, knowledge of which is indispensable for designing digital microwave devices (Kiang, 1991). Waveguides with low attenuation are required to create highly effective radio-relay, space and tropospheric-scatter communication facilities and to design measuring equipment (including that operating at the millimetre waveband and infrared). High-quality resonators are widely used in radio engineering (tunable oscillators with high frequency stability), in applied physics (wavemeters, equipment for measuring dielectric coefficients and permeabilities of substances and for micro wave heating etc.), in electronics (gyrotrons, orotrons, free electron lasers etc.) and in unique physical experiments (for instance, experiments on detecting gravitational waves).
-
2 Surface-impedance technique for the study of dissipation processes in bodies with finite conductivity
- + Show details - Hide details
-
p.
18
–36
(19)
Dissipation of electromagnetic waves in metal structures, due to their finite conductivity, is usually investigated employing impedance boundary conditions. The use of these boundary conditions allows the problem to be simplified, as the field inside the metal is not considered, which is the essence of the surface-impedance method. Penetration of microwave fields into the metals is accompanied by a marked skin effect and this stipulates the following special features, which are important for the conditions to be used in the future.It is important to note that the surface-impedance method is a fairly universal technique which covers practically all interesting cases when dissipation characteristics of microwave and millimetre-wave components are to be determined. In this Chapter, expressions of the surface impedance for such cases will be considered.
-
3 Normal modes in waveguides with losses
- + Show details - Hide details
-
p.
37
–82
(46)
This Chapter deals with the theory of regular waveguides with impedance walls. Spectral problems for normal modes have been formulated. General character istics of different types of normal modes (eigenmodes, associated modes) have been considered. Calculation of attenuation coefficients of the eigenmodes for the case of small losses is discussed. General expressions for attenuation coefficients are obtained with the aid of the perturbation technique. Applicability cases for different variants of the perturbation method are analysed in detail (energy-perturbation method, perturbation technique for solving the dispersion equation). Their modifications are described for cases when classic schemes of the method are not usable (degenerated modes of D-multiplicity, models of waveguides with unclosed infinitely thin metal surfaces). Typical principal errors in attenuation coefficient calculation are analysed. Computational peculiarities of different approaches are pointed out, as well as the expediency of their application for different cases. General principles are illustrated by specific examples (rectangular and circular waveguides, coaxial and microstrip lines).
-
4 Normal oscillations in resonators with losses
- + Show details - Hide details
-
p.
83
–122
(40)
This Chapter deals with the theory of normal oscillations in resonators with losses in the walls, and also cavities filled with inhomogeneous dissipative dispersive media. Concepts of eigenoscillations and free oscillations and corresponding Q-factors are introduced. General formulae for the Q-factors of eigenoscillations and free oscillations are derived and the difference between them, resulting from the dispersion properties of the media, is considered. The Q-factors of various resonators (cylindrical, spherical, conical, biconical and cylindrical with coaxial metal plug) are calculated. Different approaches are used to calculate Q-factor (energy-perturbation method, impedance-perturbation technique for solving the characteristic equation), and the advantages and disadvantages of each method are discussed. Comparisons are made of resonators of different shapes with respect to their Q-factor. Ways of increasing the Q-factor through optimisation of the resonator shape and filling are analysed.
-
5 Electromagnetic-wave diffraction by finitely conducting comb-shaped structures
- + Show details - Hide details
-
p.
123
–178
(56)
This chapter is dedicated to the diffraction of plane waves by finitely conducting comb-shaped structures. Finite conductivity is taken into consideration by means of impedance boundary conditions. Periodic structures of comparatively simple configuration (array of halfplanes, symmetrical and nonsymmetrical lamellar gratings, echelettes) are examined as well as structures with complicated shape profiles. Highly effective numerical methods are described for the latter which are universal in relation to the structure configuration. Approximate formulae and numerical results are given for a linear (per period) power of heat losses. Its dependence on the angle of incidence, wave length and configuration of the unit cell is analysed. For the case of H-polarisation considerable attention is paid to the effect of abnormally small dissipation (the losses in a periodical structure may be smaller than those in a smooth surface of the same material). For E-polarised waves this does not occur. The physical mechanism of the absorption anomalies is analysed, and their practical use in reducing attenuation in microwave waveguides and resonators is discussed.
-
6 Dissipation in comb-shaped structures in inhomogeneous and anisotropic media
- + Show details - Hide details
-
p.
179
–195
(17)
This Chapter deals with the following problems: (a) H-polarised plane-wave diffraction by an imperfectly conducting rectangular-groove grating with a plane-layered dielectric halfspace over it or plane-layered dielectric filling of the grooves; (b) H-polarised plane-wave diffraction by an imperfectly conducting comb shaped structure in a gyrotropic medium.
-
7 Eigenmodes in corrugated waveguides and resonators with finitely conducting walls
- + Show details - Hide details
-
p.
196
–256
(61)
This Chapter deals with the electromagnetic fields in systems with corrugated surfaces: waveguides, resonators and horns. Special attention is given to the investigation of dissipation characteristics: attenuation coefficients of different modes in corrugated waveguides and Q-factors of eigenoscillations in resonators with corrugated walls. Application of corrugated surfaces of both step shape and smooth is considered. When a period of the structure is small compared with the wavelength, a method of equivalent impedance-boundary conditions is used. Low-loss corrugated waveguides and millimetre waveband high-quality resonators with a rare spectrum of eigenoscillations based on the effect of abnormally small absorption in periodic structures are described. A rigorous method of calculation of electrodynamic characteristics of corrugated horns, which are highly effective feeds for microwave antennae, is considered.
-
Appendix 1: Shooting method and its modifications
- + Show details - Hide details
-
p.
257
–267
(11)
This chapter discusses the shooting method. By means of Galerkin's incomplete method, a number of diffraction problems can be reduced to the resolution of a system of ordinary differential equations with boundary conditions By(0) = b, Dy(T)=d; where < t < T,y,f, b and d are vector columns. To solve this boundary-value problem the shooting technique is used.
-
Appendix 2: Expressions for current-density distributions in a microstrip line with a strip of finite thickness
- + Show details - Hide details
-
p.
268
–271
(4)
The expressions for current-density distributions in a microstrip line with a strip of finite thickness is discussed. Equations are shown.
-
Appendix 3: General formulae for the coefficients α(i)nm, β(i)nm, γ(i)nm, δ(i)nm
- + Show details - Hide details
-
p.
272
–274
(3)
In this Appendix the general formulae for the coefficients in eqn. 7.73 are given. These formulae are applicable for corrugated waveguides with arbitrary shapes of the cross-section and corrugation.
-
Back Matter
- + Show details - Hide details
-
p.
275
(1)