Electrically Small Magnetic Dipole Antennas With Quality Factors Approaching the Chu Lower Bound

We investigate the quality factor
Q
for electrically small current distributions and practical antenna designs radiating the TE
₁₀
magnetic dipole field. The current distributions and the antenna designs employ electric currents on a spherical surface enclosing a magneto-dielectric material that serves to reduce the internal stored energy. Closed-form expressions for the internal and external stored energies as well as for the quality factor
Q
are derived. The influence of the sphere radius and the material permeability and permittivity on the quality factor
Q
is determined and verified numerically. It is found that for a given antenna size and permittivity there is an optimum permeability that ensures the lowest possible
Q
, and this optimum permeability is inversely proportional to the square of the antenna electrical radius. When the relative permittivity is equal to 1, the optimum permeability yields the quality factor
Q
that constitutes the lower bound for a magnetic dipole antenna with a magneto-dielectric core. Furthermore, the smaller the antenna the closer its quality factor
Q
can approach the Chu lower bound. Simulated results for the TE
₁₀
-mode multiarm spherical helix antenna with a magnetic core reach a
Q
that is 1.24 times the Chu lower bound for an electrical radius of 0.192.