The method of moments is used to solve electromagnetic boundary value problems numerically. It is known that the choice of basis functions is crucial for the numerical efficiency. Fast convergence is achieved provided the basis functions efficiently approximate the unknown function. In this paper the far field (incl. RCS) of a thin conducting square plate is calculated. Basis functions with correct edge and corner singularities are shown to greatly enhance the convergence compared to ordinary "rooftop" functions. The calculations of the matrix elements as well as the right side of the matrix equation and the scattered field are simplified by the use of a multipole technique.