A Gaussian radio wave, propagating through the ionosphere causes differential ohmic heating of electrons and subsequent quasi-linear diffusion of the plasma. The plasma duct thus formed tends to self-focus the radio wave. When the radio wave has an on-axis spike in its intensity distribution, it causes stronger local density depression and attracts more electromagnetic energy from the neighborhood, enhancing the spike. The self-focusing of the radio wave and the growth of the perturbation are coupled through the nonlinear permittivity. For RF powers corresponding to &ngr;o~cs the optimum radius of the fastest growing perturbation is ~3c/&ohgr;po and the amplification length is ~10c&ohgr;/&ohgr;2po, where &ngr;o is the amplitude of electron oscillatory velocity, cs is the sound speed, &ohgr; is the frequency of the radio wave, and &ohgr;po is the plasma frequency. ¿ American Geophysical Union 1994