  It is required to calculate the stored reactive energy of an antenna in order
to evaluate its Q factor. Although it has been investigated for a long time,
some issues still need further clarifying. The main difficulty involved is that
the reactive energy of an antenna tends to become infinitely large when
integrating the conventionally defined energy density in the whole space
outside a small sphere containing the antenna. The reactive energy can be made
bounded by subtracting an additional term associated with the radiation fields.
However, there exists no well-founded accurate definition for this additional
term that is valid for all cases. By re-checking the definition of energy
densities, a new formulation is proposed in this paper to separate the reactive
energy and the radiation energy explicitly. The clearly defined reactive energy
is bounded without necessary to subtract the additional term, and the formulae
are easy to implement.
