A modal analysis method is used to investigate scattering from conducting objects of rectangular cross-section, and to determine the variation of the radar cross-section due to the polarisation or direction of the incident wave and the effect of the mode coupling resulting from crosssectional perturbation from a circular cylinder. The traditional approach of surface integral equations in conjunction with a moment method is used to solve the problem numerically. A geometric transformation is used to select entire domain basis functions for the surface current along the cross-section. Subdomain basis functions are used to represent the current component in the plane orthogonal to the cross-section. Owing to the geometric structure of the object the current modes are coupled. The degree of coupling and convergence of the modes are investigated. Representative results for objects of square and rectangular cross-sections are presented and discussed.