Representation of Electromagnetic Energy Around Antennas: Current research and future directions

In this article, we reconsider the subject of spatiotemporal electromagnetic (EM) energy around the antenna near field from a fundamental perspective and discuss the use of radiation quality factors (Q factors) in this context. After a thorough literature review, we demonstrate how minimization of the Q factor and maximization of the directivity Q ratio have paved the way for the optimal design of electrically small antennas (ESAs) through the years. Then, we discuss the inherent limitations of this frequency domain Q factor approach, especially for analysis of multiresonant, ultrawideband (UWB), and nonresonant leaky-wave antennas. We further point out that the Q factor is a circuit theoretic construct valid for the steady-state, time-harmonic excitation of antennas and hence not equipped to 1) generate spatial maps of near-field nonpropagating/localized antenna energy and 2) quantify the effects of switching/pulse-like excitations on the energy dynamics.