Phenomena of coupling of wave paraxial beam and quasi-surface wave at perfectly conducting plane with segment of spatially oscillating surface impedance is considered. Integral equation for perturbed current distribution is obtained. Method of moments is used. Solution was expanded in terms of long harmonic subdomain (middle-domain) basis functions. Two types of discretization were applied. Advantages of complicated test functions in the form of harmonic superpositions of delta-functions were demonstrated.