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An electrically small antenna is defined as an antenna with a volume smaller than a radian sphere defined by H. A. Wheeler.〔H. A. Wheeler, "The Radiansphere around a Small Antenna," Proceedings of the IRE, vol. 47, pp. 1325-1331, 1959.〕 : where ''r'' is the radius of a sphere, and λ is the free space wavelength. The far-field radiation pattern of an antenna is the sum of its near-field spherical modes, expressed using Legendre functions and spherical Bessel functions. In its simplest form, it is an omnidirectional radiation pattern with no variation in the azimuth plane. When the antenna becomes electrically small, the propagating modes are replaced by evanescent modes with high Q, where : In short, the maximum bandwidth of an electrically small antenna is regulated by its maximum dimension enclosed within a sphere of radius . The difficulties of designing an electrically small antenna includes: : *impedance matching, : *insertion loss from high current density flowing on a non-perfect conductor, resulting in joule heating, and : *a small radiation aperture with low radiation efficiency. == History == The theoretical limitation of an electrically small antenna and its bandwidth was first investigated by L. J. Chu.〔L. J. Chu, "Physical Limitations on Omni-Directional Antennas," J. Appl. Phys., Vol. 9, pp. 1163-1175, 1948.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Electrically small antenna」の詳細全文を読む スポンサード リンク
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