德国卡尔斯鲁厄大学高级无线通信讲义

ARC_Antennas2 Antennas
2.1 Introduction
In electrical engineering, information transfer means energy transfer. This is normally achieved by guiding structures such as two wire transmission lines, coaxial transmission lines, hollow or dielectric waveguides. Let us assume that a guiding structure 1 E × H consists of an ideally conducting material. In this case, the power density S = 2 (assuming ejωt dependence) is directed along the guiding structure and lies completely outside the conductor. On the other hand, e.g. a plane wave traveling in +z -direction E = E0 ejβz ex E0 jβz e ey H= Z0 (2.1a) (2.1b)

with Z0 = 0 /ε0 ≈ 377 as the free space wave impedance satises Maxwell’s Equations and doesn’t need any guiding structures. The task of an antenna is to allow energy transfer without guiding structures a