In order to make antenna have a beautiful feature and a reliable structure, with coattails logarithmic antenna structure is used as the research object, an approximate method for calculating the structure size of logarithmic antenna is proposed. The requirement for the given dipole sag is met in the following calculations. First of all, tension value of each oscillator are obtained by using small sag flexible cable theory, and then by analyzing stressed conditions of antenna cable in the vertical plane, the coordinates of each hanging point and the tension of both ends for cable are calculated through the torque balance. If the tension value of each oscillator and cable is greater than the corresponding required value, then the cable is re-analyzed by adjusting the oscillator sag and the giving the shortest tail until the tension values are within the allowable range, and then the iterative calculation is ended. The length of antenna cable, tail, and cable pre-tension are obtained, thus the data give the support to antenna structure design, material selection, and engineering applications. Finally, through the verification of practical engineering project, the analysis results obtained by using the method have an important value for engineering applications. This method is applicable to the analysis and calculation of cable for vertical wire antenna acted by focus load with different size, distance, and directions.