Time-domain near-field analysis of short-pulse antennas - Part I: Spherical wave (multipole) expansion

The radiation from a time-dependent source distribution in free-space is an alyzed using time-domain (TD) spherical wave (multipole) expansion. The mul tipole moment functions are calculated from the time-dependent source distr ibution. The series convergence rate in the near and far zone and the bound s on the near-zone reactive field are determined as functions of the source support and of the pulse length. The formulation involves a spherical tran smission line representation that can be extended to more general spherical configurations. This formulation also describes the field and energy trans mission mechanisms in a physically transparent fashion that will be used in a companion paper to define and explore fundamental concepts such as TD re active energy and Q and to derive bounds on the antenna properties. Finally , the concepts discussed above are demonstrated numerically for pulsed radi ation by a circular current disk.