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A simple method is advanced for calculating the distant field of a symmetrical center-driven antenna of known effective cross section. This is accomplished by introducing two fictitious antennas carrying currents of simple analytical form and of such amplitude and phase as to approximate closely the actual distribution of current. In the case of thick antennas, this departs significantly from the sinusoidal distribution ordinarily assumed. It is demonstrated that for antennas of moderate thickness the shape of the vertical field pattern differs only very slightly from that for an indefinitely thin antenna carrying a simple sinusoidal current. The magnitude of the field of the actual antenna differs by only a few per cent from that of the indefinitely thin antenna if the maximum value of the square root of the sum of the squares of the quadrature components of the current in the former is made equal to the maximum sinusoidal current in the latter. The phase of the field for the actual antenna differs from that of the indefinitely thin antenna. The cross section of a cylindrical antenna in which the magnitude of the current approximates the measured distribution of current along on an actual antenna is defined to be the effective cross section of the actual antenna.