  Though commonly used to calculate Q-factor and fractional bandwidth, the
energy stored by radiating systems (antennas) is a subtle and challenging
concept that has perplexed researchers for over half a century. Here, the
obstacles in defining and calculating stored energy in general electromagnetic
systems are presented from first principles as well as using demonstrative
examples from electrostatics, circuits, and radiating systems. Along the way,
the concept of unobservable energy is introduced to formalize such challenges.
Existing methods of defining stored energy in radiating systems are then
reviewed in a framework based on technical commonalities rather than
chronological order. Equivalences between some methods under common assumptions
are highlighted, along with the strengths, weaknesses, and unique applications
of certain techniques. Numerical examples are provided to compare the relative
margin between methods on several radiating structures.
