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A mathematical model for determining the effects of a small sinusoidal roughness on horizontally polarized radio waves, propagating over large distances in a horizontally homogenous tropospheric duct, is presented. The wave equation is solved for the appropriate boundary conditions. The eigenvalues of the solution are the roots of an infinite determinant. An effective reflection coefficient is derived used with the infinite determinant to obtain an 'equivalent zero mode matrix', from which the eigenvalues can be obtained. A perturbation method is then used to derive a computationally efficient means of calculating the eigenvalues from tbe equivalent matrix. Examples that demonstrate that the attenuation due to the roughness can be significant are provided. Keywords: Electromagnetic propagation; Rough-earth ducting; Rough boundary waveguide; Surface duct. (edc)