Observable stored energies of electromagnetic systems

Abstract The conventional definition of quality factor Q in terms of time-average stored energy is widely assumed to be a measure of the input bandwidth of any ordinary electromagnetic system. But the extent to which this assumption is true has never been established. In the case of all radiating systems, for example, it is known to fail completely. It can be made true quite generally, however, by including only those parts of the total time-average stored energy that give a physically observable contribution to the input bandwidth. Explicit formulas for these newly defined observable stored energies are developed that are valid for any electromagnetic system that is linear, passive, and time-invariant.