ABSTRACT
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 3Hz 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).
