In the design of a directional array the engineer is confronted with the solution of a problem that is laborious because of the many parameters to be considered. The variables that enter the problem are spacing, phasing, radiated field, and orientation for each antenna involved. In particular, for an array of 3 elements there are 9 parameters when one of the elements is considered to be at the origin. This paper describes a mechanical calculator that eliminates a major part of the tedious work involved in the calculation of the horizontal pattern of a 2- or 3-element array. The calculator is of equal use in the determination of the vertical patterns when the antenna heights are equal. An analysis is made to show that the results given by the calculator are solutions to the standard equation. The accuracy of the results are also considered.