A new mehod is described for calculating the field produced by a rectangular array of large charged cylinders. The method involves finding an arrangement of sources (line charges) which satisfy, with acceptable accuracy, the boundary conditions in the unit cell surrounding the cylinder. Two cases are described: an array in which alternate rows of cylinders are charged positively and negatively and an array in which alternate cylinders in each axial direction are oppositely charged. Computer plots of fields calculated for typical arrangements of line charges are given, which are used to determine the relationships between proportions of the unit cell, the values and positions of charges, the cylinder radius, and for estimation of accuracy of the simulation. It is shown that accuracy is acceptably high for the different proportions of unit cells and cylinder radii studied.