Abstract: The eigen permeabilities and eigen waves of the basic transmission modes are deduced,which play intrinsical roles in the propagation of plane waves in an unreciprocal medium. The eigen permeability correspond to the propagation coefficient of eigen waves,and all transmission modes can be considered as the linear combinations of these eigen waves.It is also demonstrated that the eigen wave is of ellipfical polarizati in general.

Key words: Eigen permeability, Eigen wave, Arbitrary direction, Unreciprocal medium