Minimum Q Electrically Small Spherical Magnetic Dipole Antenna - Practice

Practical aspects of applying a magnetic core to approach the Chu lower bound for the radiation Q factor of an electrically small magnetic dipole antenna are considered. It is shown that although a magnetic core does reduce the Q factor, its effect is not as strong as predicted by Wheeler. This is due to the fact that a finite size magnetic core supports multiple internal resonances, which spoil the Q factor also away from exact resonance frequencies; and in a worst case they can even significantly increase Q.

The resonances in question are not only those of the TE10 spherical mode, but also resonances of all other modes that are not sufficiently suppressed in the antenna. Numerical results for a 4-arm spherical helix antenna filled with magnetic material demonstrate the destroying effect of the parasitic TM11 mode on the antenna Q factor.

Theoretical considerations as well as numerical results show that in a given range of magnetic permeabilities away from core resonances there is an optimum permeability for which the Q factor is lowest. In the given example, the antenna of the size ka~0.254 yields the Q ratio 1.28 times above the Chu lower bound.