The Spatial Distributions of Radiation Emitted from a Sinusoidal Current Filament and a Dipole Antenna

Abstract

While the analytical and numerical tools for determining the basic properties of a variety of antenna types have been long-established, there remains some continuing curiosity about how electromagnetic radiation is launched by such a simple antenna as a dipole. The following article discusses this problem in both the frequency domain and time domain. The sinusoidal current filament (SCF) is investigated first as a prototype of a wire dipole. The length-wise distribution of radiated power for the SCF is obtained from the distributed radiation resistance of Schelkunoff and Feldman, the induced electromotive force (IEMF) method, and the far-field analysis of radiation sources (FARS) developed by the author. The FARS approach is next used to analyze a frequency-domain numerical model of a dipole antenna, producing results similar to those for the SCF for a dipole of near-zero radius. Differentiating the decaying on-surface Poynting vector (PV) produces results comparable to those from FARS to explicitly demonstrate the power loss caused by radiation of the propagating current and charge. The lobed distributed radiated power is shown to be closely correlated with the square of the dipole current, confirming the cause of the radiation to be due to a partially reflected charge as the current and charge form standing waves on the dipole. Application of a time-domain version of FARS yields a smoothed length-wise distribution of radiated energy as opposed to the lobed variation of the frequency domain.