tailieunhanh - Giai đoạn mảng anten P9

A useful operational definition of antenna array superdirectivity (formerly called supergain) is directivity (see Chapter 2) higher than that obtained with the same array length and elements uniformly excited (constant amplitude and linear phase). Excessive array superdirectivity inflicts major problems in low radiation resistance (hence low efficiency), sensitive excitation and position tolerances, and narrow bandwidth. | Phased Array Antennas. Robert C. Hansen Copyright 1998 by John Wiley Sons Inc. ISBNs 0-471-53076-X Hardback 0-471-22421-9 Electronic CHAPTER NINE Superdirective Arrays HISTORICAL NOTES A useful operational definition of antenna array superdirectivity formerly called supergain is directivity see Chapter 2 higher than that obtained with the same array length and elements uniformly excited constant amplitude and linear phase . Excessive array superdirectivity inflicts major problems in low radiation resistance hence low efficiency sensitive excitation and position tolerances and narrow bandwidth. Superdirectivity applies in principle to both arrays of isotropic elements and to actual antenna arrays composed of nonisotropic elements such as dipoles. Probably the earliest work on the possibility of superdirectivity was by Oseen 1922 see Bloch et al. 1953 for a list of early references. A limited endfire superdirectivity using a monotonic phase function was accomplished by W. W. Hansen and Woodyard 1938 . Another early contributor was Franz 1939 . Schelkunoff 1943 in a classic paper on linear arrays discussed among other topics array spacings less than X 2 showing how equal spacing of the array polynomial zeros over that portion of the unit circle represented by the array gives superdirectivity. The field received wide publicity when La Paz and Miller 1943 purported to show that a given aperture would allow a maximum directivity and when Bouwkamp and De Bruijn 1946 showed that they had made an error and that there was no limit on theoretical directivity. Thus the important theorem A fixed aperture size can achieve in theory any desired directivity value. This theorem is now widely recognized but the practical implications are less well known. Bloch et al. 1960 say that the theorem has been rediscovered several times the practical limitations of superdirectivity occur as a surprise to systems engineers and others year after year In 1946 a burst of wartime research .