The radiation field of an asymmetrically driven thin linear antenna

This paper derives a compact algebraic expression for the radiation field of a thin linear antenna of arbitrary length and arbitrary driving-point location. The vector-potential method is used to calculate the far field from a sum of an assumed generalized asymptotic form of current which neglects radiation and a phase quadrature current which accounts for the radiated power. The radiation field contains both real and imaginary parts. If the driving point is moved to the center of the antenna, the imaginary components vanish and the field becomes the text book expression for the far field of a center-driven, thin linear antenna. The expression for the radiation field clearly shows that a driving point located away from the center of the antenna causes the imaginary components to combine in such a way that the field pattern becomes asymmetric. The calculated far fields of several thin, linear asymmetrically-driven antennas are compared with measured field patterns found in the literature and with measurements made in this study. Reasonably good agreement is found between the measured and calculated fields.