%0 Journal Article
%T Absorption Efficiency and Physical Bounds on Antennas
%A Mats Gustafsson
%A Marius Cismasu
%A Sven Nordebo
%J International Journal of Antennas and Propagation
%D 2010
%I Hindawi Publishing Corporation
%R 10.1155/2010/946746
%X The all spectrum absorption efficiency appears in the physical bounds on antennas expressed in the polarizability dyadics. Here, it is shown that this generalized absorption efficiency is close to 1/2 for small idealized dipole antennas and for antennas with a dominant resonance in their absorption. Also, the usefulness of this parameter is analyzed for estimation of antenna performance. The results are illustrated with numerical data for several antennas. 1. Introduction A new set of physical bounds on antennas was introduced in [1每5]. These bounds relate the performance of the antenna to the electro- and magnetostatic polarizability dyadics of a circumscribing geometry. This generalizes the classical bounds by Chu [6] for spherical geometries to geometries of arbitrary shape. The new bounds are valid for lossless and linearly polarized [1每4] and elliptically polarized [5] antennas. Moreover, the approach can be used to estimate the performances of many small antennas if the polarizabilities of the antennas are used instead of the circumscribing geometries [2每4]. The only parameter in the bound that depends on the dynamic properties of the antenna is the generalized (or all spectrum) absorption efficiency, . This is the generalization of the frequency-dependent absorption efficiency analyzed in [7] given by integration of the absorbed and total power, independently, over all wavelengths. In [1每4], it is demonstrated that is close to for many small antennas that are connected to a frequency independent resistive load and matched at their first resonance. This is motivated by the minimum scattering property that small-matched antennas often possess, that is, they scatter as much power as they absorb at the resonance frequency giving an absorption efficiency of at the resonance frequency [7, 8]. Here, it is shown that small idealized dipole antennas with a dominant first single resonance have an all spectrum absorption efficiency . The region around the resonance is minimum scattering but the contributions from regions away from the resonance scatter slightly more power than is absorbed giving a generalized (all spectrum) absorption efficiency close to but less than . Minimum scattering is a property that many non electrically small resonant antennas also possess. Numerical simulation results of common antennas, both electrically small and not small, verify the theoretical results. 2. Absorption Efficiency The physical bounds analyzed in [1每4] are derived for single port, linearly polarized, reciprocal, and lossless antennas with the reflection
%U http://www.hindawi.com/journals/ijap/2010/946746/