Time-domain near-field analysis of short-pulse antennas -: Part II:: Reactive energy and the antenna Q

The time-domain (TD) multipole expansion, developed in the first part of this two part sequence, is extended here to analyze the power-flow and energy balance in the vicinity of a pulsed antenna, Using the spherical transmission line formulation, we derive expressions for the pulsed power-flow and energy and identify the radiative and the reactive constituents. For time-harmonic fields, the reactive concepts are well understood in terms of the stored energy, but this interpretation is not applicable for short pulse fields where there is no stored energy, By considering the TD energy balance, we clarify the transition of the near-zone pulsed reactive energy to the radiation power and show that the pulsed reactive energy discharges back to the source once the pulse has been radiated. We thus introduce a TD Q factor that quantifies the radiation efficiency, In particular we show that super resolution using short-pulse fields involves large TD reactive energies and Q and is, therefore, not feasible. The general TD concepts discussed are demonstrated through a numerical example of radiation from a circular dish carrying a pulsed current distribution.