Reactive, localized, and stored energies: The fundamental differences and proposals for new experiments

The question of energy is fundamental in electromagnetics and has been around since the inception of the modern theory of electromagnetic fields by Faraday and Maxwell. However, the topic remains shrouded with serious conceptual difficulties in both the physical literature [1] and the engineering side, see for example the recent antenna near field theory and the open questions left there [2]. The present paper aims at providing a fresh view on the topic taking into account recent progress in both theoretical and applied research. We highlight some subtle and not well-acknowledged differences between three types of basic energy processes in general antenna systems: reactive energy, localized energy, and stored energy. The explication will rely on careful analysis of the way the energy concept is used in various applications, such as matching (reactive energy), near field control and functions (localized energy), and energy harvesting and retrieval (stored energy).