chapt14 - CHAPTER 14 WAVEGUIDE AND ANTENNA FUNDAMENTALS As...

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CHAPTER 14 WAVEGUIDE AND ANTENNA FUNDAMENTALS As a conclusion to our study of electromagnetics, we investigate the basic prin- ciples of two important classes of devices: waveguides and antennas. In broad definitions, a waveguide is a structure through which electromagnetic waves can be transmitted from point to point, and within which the fields are confined to a certain extent. An antenna is any device that radiates electromagnetic fields into space, where the fields originate from a source that feeds the antenna through a transmission line or waveguide. The antenna thus serves as an interface between the confining line and space when used as a transmitterÐor between space and the line when used as a receiver. In our study of waveguides, we will first take a broad view of waveguide devices, to obtain a physical understanding of how they work and the conditions under which they are used. We will next explore the simple parallel-plate wave- guide and study the concept of waveguide modes and the conditions under which these will occur. We will study the electric and magnetic field configurations of the guided modes using simple plane wave models and through use of the wave equation. We will then study more complicated structures, including the rectan- gular waveguide and the dielectric slab guide. Our study of antennas will include the derivation of the radiated fields from an elemental dipole, beginning with the retarded vector potentials that we stu- died in Chap. 10. We will address issues that include the efficiency of power radiation from an antenna, and the parameters that govern this. 484
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14.1 BASIC WAVEGUIDE OPERATION Waveguides assume many different forms that depend on the purpose of the guide, and on the frequency of the waves to be transmitted. The simplest form (in terms of analysis) is the parallel-plate guide shown in Fig. 14.1. Other forms are the hollow-pipe guides, including the rectangular waveguide of Fig. 14.2, and the cylindrical guide, shown in Fig. 14.3. Dielectric waveguides, used primarily at optical frequencies, include the slab waveguide of Fig. 14.4 and the optical fiber, shown in Fig. 14.5. Each of these structures possesses certain advantages over the others, depending on the application and the frequency of the waves to be transmitted. All guides, however, exhibit the same basic operating principles, which we will explore in this section. To develop an understanding of waveguide behavior, we consider the parallel-plate waveguide of Fig. 14.1. At first, we recognize this as one of the transmission line structures that we investigated in Chap. 13. So the first ques- tion that arises is: How does a waveguide differ from a transmission line to begin with? The difference lies in the form of the electric and magnetic fields within the line. To see this, consider Fig. 14.6 a , which shows the fields when the line operates as a transmission line. A sinusoidal voltage wave, with voltage applied between conductors, leads to an electric field that is directed vertically between the conductors as shown. Since current flows only in the
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This note was uploaded on 03/30/2010 for the course EE 2317 taught by Professor Wilton during the Spring '10 term at University of Houston.

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chapt14 - CHAPTER 14 WAVEGUIDE AND ANTENNA FUNDAMENTALS As...

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