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Displacing a linear source exciting the waveguide along an arbitrary fixed direction to infinity one can create the conditions in which the wave-guide is excited by a plane wave. The nature of the excited field in a wave-guide is complicated. On the upper plane of the waveguide x = +l one always deals with the radiation mode whereas on the lower boundary of the waveguide x = -l one can deal with the radiation mode or the surface-type wave.