On the Lower Bound of the Radiation Q for Electrically Small Antennas

The fundamental question of the lower bound on the radiation Q of an electrically small antenna is of practical importance because of its relationship to the antenna bandwidth. Previous works predict a lower bound on the radiation Q that is usually too low and, hence, a bandwidth that can be optimistically large. This paper addresses why this is so and offers a new prediction for a realizable lower bound on the radiation Q. This new prediction is based on the far-field pattern, in both the visible and invisible spatial regions, in contrast to previous works based upon a near-field modal approach. Results for a linear dipole, bow-tie, and end-loaded dipole are presented to illustrate the validity of the lower bound presented herein. Radiation Q can be related to bandwidth provided the Q is adequately large. Implicit is the presence of a matching network as a part of the antenna system. Both the losses in the antenna and the losses in the matching network have an effect on the system bandwidth, the system efficiency and the system Q, of which the radiation Q is a part. These various relationships are also discussed.