The concept of linear conversion of radio waves into electrostatic (ES) waves is adapted to ionospheric radio heating experiments. If is identified as access to the plasma resonance through the radio window. By means of existing heating facilities, large concentrations of electrostatic wave energy can be generated. The ES waves are confined to a restricted region in space, horizontally displaced in relation to the vertical over the heating transmitter along the magnetic meridian, the displacement being southward on the northern hemisphere. Ray-tracing studies of the location of the resonance region relative to the transmitter are presented, and the horizontal dimension is estimated.