Lower Bounds on Q for Finite Size Antennas of Arbitrary Shape

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.