Simple expressions are derived for determining the number of significant cylindrical and spherical wave harmonics in the field of any antenna. They depend on the maximum radial extentaof the source and the distancerto the field point. The expressions show that very close to the source the number of significant harmonics in the field is potentially large, but it decreases with distance from the source. The number of significant harmonics, in general, approaches existing guidelines only for electrically large antennas and in the far field. The evaluated amplitudes of the harmonics in the fields of practical antenna models show that the expressions derived here give reasonably close bounds while the currently used guideline is not generally correct.
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Ojeba et al. (1979) studied this question.
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