Stored Electromagnetic Energy and Quality Factor of Radiating Structures
  This paper deals with the old yet unsolved problem of defining and evaluating the stored electromagnetic energy - a quantity essential for calculating the quality factor, which reflects the intrinsic bandwidth of the considered electromagnetic system. A novel paradigm is proposed to determine the stored energy in the time domain leading to the method, which exhibits positive semi-definiteness and coordinate independence, i.e. two key properties actually not met by the contemporary approaches. The proposed technique is compared with two up-to-date frequency domain methods that are extensively used in practice. All three concepts are discussed and compared on the basis of examples of varying complexity, starting with lumped RLC circuits and finishing with realistic radiators. 
Miloslav Capek, Lukas Jelinek, Guy A. E. Vandenbosch
physics.class-ph