Physical Bounds of Antennas

Design of small antennas is challenging because fundamental physics limits the antennas performance. Physical bounds provide basic restrictions on the antenna performance solely expressed in the available antenna design space. These bounds offer antenna designers a priori information about the feasibility of antenna designs and a figure of merit for different antenna designs. Here, an overview of physical bounds on antennas and the development from circumscribing spheres to arbitrary shaped regions and embedded antennas are presented. The underlying assumptions for the methods based on circuit models, mode expansions, forward scattering, and current optimization are illustrated and their pros and cons are discussed. The physical bounds are compared with numerical data for several antennas