tailieunhanh - Fundamentals of Engineering Electromagnetics - Chapter 6

Một đường dây truyền tải là một hệ thống điện hướng dẫn cho truyền dữ liệu điểm-điểm hiệu quả của tín hiệu điện (thông tin) và sức mạnh. Kể từ khi sử dụng sớm nhất của nó trong điện báo Samual Morse trong những năm 1830, đường dây tải điện đã được sử dụng trong các loại khác nhau của hệ thống điện bao gồm một loạt tần số và các ứng dụng. Ví dụ về các ứng dụng trực tuyến truyền phổ biến bao gồm truyền hình cáp, dây chuyền thức ăn ăng-ten, dây cáp điện thoại, cáp mạng. | 6 Transmission Lines Andreas Weisshaar Oregon State University Corvallis Oregon . INTRODUCTION A transmission line is an electromagnetic guiding system for efficient point-to-point transmission of electric signals information and power. Since its earliest use in telegraphy by Samual Morse in the 1830s transmission lines have been employed in various types of electrical systems covering a wide range of frequencies and applications. Examples of common transmission-line applications include TV cables antenna feed lines telephone cables computer network cables printed circuit boards and power lines. A transmission line generally consists of two or more conductors embedded in a system of dielectric media. Figure shows several examples of commonly used types of transmission lines composed of a set of parallel conductors. The coaxial cable Fig. consists of two concentric cylindrical conductors separated by a dielectric material which is either air or an inert gas and spacers or a foamfiller material such as polyethylene. Owing to their self-shielding property coaxial cables are widely used throughout the radio frequency RF spectrum and in the microwave frequency range. Typical applications of coaxial cables include antenna feed lines RF signal distribution networks . cable TV interconnections between RF electronic equipment as well as input cables to high-frequency precision measurement equipment such as oscilloscopes spectrum analyzers and network analyzers. Another commonly used transmission-line type is the two-wire line illustrated in Fig. . Typical examples of two-wire lines include overhead power and telephone lines and the flat twin-lead line as an inexpensive antenna lead-in line. Because the two-wire line is an open transmission-line structure it is susceptible to electromagnetic interference. To reduce electromagnetic interference the wires may be periodically twisted twisted pair and or shielded. As a result unshielded twisted pair UTP cables