How an antenna launches its input power into radiation: The pattern of the Poynting vector at and near an antenna

Abstract

I address the question of whether the seat of the power radiated by an antenna made of conducting members is distributed over the arms of the antenna according to −J∙E, where J is the current density and E is the electric field produced by the source. Poynting’s theorem permits only a global identification of the total input power, usually from a localized generator, with the total power radiated to infinity, not a local correspondence of −J∙Ed3x with a specific radiated power, r2S∙r̂dΩ. I describe a model antenna consisting of two perfectly conducting hemispheres of radius a separated by a small equatorial gap across which occurs the driving oscillatory electric field. The fields and surface current are determined by solution of the boundary value problem. In contrast to the first approach (not a boundary value problem), the tangential electric field vanishes on the metallic surface. There is no radial Poynting vector at the surface. Numerical examples are shown to illustrate how the energy flows from the input region at the gap and is guided near the antenna by its arms until it is launched at larger r∕a into the radiation pattern determined by the value of ka.