  The main goal of this work is to theoretically investigate the behavior of an
electrically small antenna enclosed in a concentric sphere. The Greens function
analysis is applied to characterize the input impedance of a concentric
resonator excited by a dipole located at its center. The method of moments
(MoM) with Galrekin's procedure is used to determine the current distribution
over the source excitation and hence the input impedance. The behavior of
quality factor (Q) and bandwidths of the antenna is studied with the use of
input impedance as a function of frequency. We illustrate that by embedding a
dipole antenna inside a core-shell structure, with magnetic shell and
dielectric core, a Q as low as the Chu limit can be approached. The obtained
observations demonstrate how a resonator composed of magnetic shells can
provide electrically small antennas with high bandwidths performance.
