The consideration of electromagnetic wave propagation in irregular waveguiding structures usually assumes either smooth varying of medium parameters [1] or their rigorously periodical perturbation [2, 3]. Combination of smooth and oscillating parameter perturbations leads to additional peculiarities of Bragg scattering [4]. Presence of gyrotropy of permittivity or permeability causes coupling of waves with different linear polarizations.

In the present paper using complex form [4] of asymptotic method of Krylov, Bogoliubov and Mitropolsky (KBM) [5], mutual transformation of waves in open guiding structures has been theoretically considered. Influence of surface waves on scattering by a surface with spatially oscillated surface impedance has been analyzed. Additional phenomena of nonreciprocal wave coupling caused by variation of surface gyrotropy described by variable surface impedance tensor have been investigated.