tailieunhanh - Fundamentals of Engineering Electromagnetics - Chapter 8

Bức xạ điện từ là một trong các hình thức chủ yếu của truyền đạt thông tin từ một điểm đến khác từ người này đến máy tính, người điện thoại, máy tính đến điện thoại và phát sóng đài phát thanh để thu radio. Bức xạ được sử dụng trong các hệ thống thông tin liên lạc thường nằm trong khoảng tần số từ tần số rất thấp (ELF) quang học và tia cực tím (UV) tần số. Ví dụ, bức xạ ELF (băng tần số 3 Hz đến 3 kHz) được sử dụng trong tuyên truyền qua đất. | 8 Antennas Fundamentals David Thiel Griffith University Nathan Queensland Australia . INTRODUCTION TO RADIATION Electromagnetic radiation is one of the principal forms of conveying information from one point to another from person to person computer to computer telephone to telephone and broadcast radio station to radio receiver. The radiation used in these communications systems usually lies in the frequency range from extremely low frequencies ELF to optical and ultraviolet UV frequencies. For example ELF radiation frequency band 3 Hz to 3 kHz is used in through-earth propagation and telephone modems. Optical and UV frequencies are commonly used with optical fibers and sometimes in open-air links. Electromagnetic radiation can be trapped and directed along conductive wires transmission lines dielectric filled conducting pipes wave guides and in dielectric pipes sheathed with dielectric materials with a lower dielectric constant optical fibers . In many cases it is desirable to have a wireless EM link so that the radiation is unguided and will generally follow a line-of-sight path . a geometrical optics path . In the radio-frequency RF -microwave-frequency range antennas are often used to launch and focus the radiation to a limited beam width so that the signal to noise ratio at the receiver is maximum and the interference to other wireless links in the same frequency band is minimized. An antenna is therefore a device that converts confined radiation from a transmission line or waveguide into an unguided but directed electromagnetic wave in the ambient medium often but not always air . While electromagnetic waves can propagate along the interface between media . surface waves and in waveguides . TE and TM waves in such a way that the electric and magnetic fields are not perpendicular to the direction of propagation in most cases the free-space radiation is in the form of a transverse electromagnetic TEM wave. For example for a TEM wave if we choose .