Using high permeability shells to improve the Q of electrically small electric-dipole antennas

Abstract

The lower bounds on the radiation quality factor (Q) of electrically small antennas were first derived by Chu [1] for a spherical surface circumscribing an antenna under the assumption that there is no stored energy inside the sphere except possibly for the stored energy needed to tune to zero the reactance produced by the stored energy outside the sphere. It is generally assumed there will be additional stored energy inside an actual antenna that will raise the Q above the Chu lower bound. Wheeler [2] was the first to observe that, for the case of a small spherical magnetic-dipole antenna with electric currents confined to its surface, filling the antenna volume with a material of infinite magnetic permeability would reduce the stored magnetic energy inside the sphere to zero, enabling the Chu lower bound to be achieved theoretically. Wheeler offered no analogous method for reducing the internal stored electric energy of an electrically small spherical electric-dipole antenna.