Teaching antenna radiation from a time-domain perspective

Abstract

Radiation from a simple wire antenna, such as a dipole, is a topic discussed in many courses on electromagnetism. These discussions are almost always restricted to harmonic time dependence. A time-harmonic current distribution is assumed on the wire, and the time-harmonic radiated field is determined. The purpose of this paper is to show that simple wire antennas with a general excitation, e.g., a pulse in time, can be analyzed easily using approximations no worse than those used with time-harmonic excitation, viz. an assumed current distribution. Expressions are obtained for the electromagnetic field of the current that apply at any point in space (in the near zone as well as in the far zone). The analysis in the time domain provides physical understanding not readily available from the time-harmonic analysis. In addition, an interesting analogy can be drawn between the radiation from these antennas when excited by a short pulse of current and the radiation from a moving point charge.