  New expressions are derived to calculate the Q factor of a radiating device.
The resulting relations link Q based on the frequency change of the input
impedance at the input port (Qx, Qz) with expressions based solely on the
current distribution on an radiating device. The question of which energies of
a radiating system are observable is reviewed, and then the proposed Q factor
as defined in this paper is physical. The derivation is based on potential
theory rather than fields. This approach hence automatically eliminates all
divergent integrals associated with electromagnetic energies in infinite space.
  The new formulas allow us to study the radiation Q factor for antennas
without feeding (through e.g. Characteristic Modes) as well as fed by an
arbitrary number of ports. The new technique can easily be implemented in any
numerical software dealing with current densities. To present the merits of
proposed technique, three canonical antennas are studied. Numerical examples
show excellent agreement between the measurable Qz derived from input impedance
and the new expressions.
