  The problem of the lower bound on the radiation Q for an arbitrarily shaped
finite size antenna of non-zero volume is formulated in terms of equivalent
electric and magnetic currents densities distributed on a closed surface
coinciding with antenna exterior surface. When these equivalent currents
radiate in free space, the magnetic current augments the electric current, so
that the fields interior to the surface vanish. In contrast to approaches based
solely on electric currents, the proposed technique ensures no stored energy
interior to the antenna exterior surface, and thus, allows the fundamental
lower bound on Q to be determined. To facilitate the computation of the bound,
new expressions for the stored energy, radiated power, and Q of coupled
electric and magnetic source currents in free space are derived.
