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 clarification. 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 is usually made to be bounded by
subtracting an additional term associated with the radiation fields.
However, there exists no well-accepted accurate definition for this
additional term that is valid for all cases. By re-checking the
definition of reactive energies, a new formulation is proposed in this
paper which can separate the reactive energy and the radiation energy
explicitly based on source-potentials. The clearly defined reactive
energy is bounded without necessity to subtract that additional term,
and the resultant formulae are easy to implement.