The modulation of the radar backscatter by long ocean waves can, to first order, be described by a linear modulation transfer function (MTF). The complex MTF can be separated into a part describing purely geometrical effects and the `hydrodynamic MTF' resulting from the modulation of short surface waves by longer waves. According to previous theories, the hydrodynamic MTF should be independent of the radar polarization. Experimental results show that this is not valid. Furthermore, the measured hydrodynamic MTFs for high radar frequencies (e.g., X-band) are often larger than expected. We show that these effects can be basically explained by a three-scale composite surface model, which takes into account the second-order modulation of the radar backscatter by intermediate-scale surface waves. Furthermore, the proposed model includes a modulation of the wind stress over the long surface waves.