The Role of Reactive Energy in the Radiation by a Dipole Antenna

The spatio-temporal distribution of the near field around an antenna is of fundamental importance in understanding its radiative properties, since it can affect the radiated power flow emitted by the device. In this work, we consider the simplest case of a harmonic point dipole antenna activated at some point in time, and study the distribution of the energy flow velocity and reactive energy in space and time. We discuss how the reactive energy building up around the radiator affects quite strongly the flow of energy in the vicinity of the dipole, while in the far field, the expected radiation features are restored. The reactive energy in the near field decelerates or even inverts the overall energy flow, resulting in partial standing waves near the feed. We also investigate the transient response of the stored reactive energy and identify the convergence to steady state, which is demonstrated through time-averaged quantities. Our results shed interesting observations on the radiation properties of antennas in their near field, as time progresses.