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The role of mode coupling between ordinary, O, and extraordinary, X, mode waves in the vicinity of the radio wave plasma frequency is examined. For propagation in a stratified inhomogeneous ionosphere the roots of the Booker quartic indicate that coupling is possible over a narrow height range. The amount of coupling depends primarily upon both the launch angle of the radio wave and the vertical variability or gradient of the electron density profile. The basic theory underlying this phenomena is reviewed, and roots of the quartic and full wave solutions are presented to demonstrate the theory. Of particular importance is the fact that most of the energy transferred to the X mode will be deposited within the ionosphere through deviative absorption and plasma wave generation. Implications of this coupling phenomenon for ionospheric high-power radio wave modification and its relationship to several experimental observations are noted.