The nonlinear features of type-1 radar echoes were studied by a two-dimensional (2D) simulation of saturation the Farley-Buneman (FB) wave. The behavior of this FB wave in a plane perpendicular to the Earth's magnetic field was simulated with a two-fluid code in which electron inertia was discounted while ion inertia was retained. It showed that the appearance of secondary waves propagating vertically and obliquely as the primary horizontal FB wave saturates. The secondary waves originating from nonlinear saturation process will construct the evolution of 2D modes which can be observed by oblique radar beams. We carried out the statistical analysis of projection phase velocities of 2D modes along oblique radar beam at different radar elevation angles. The result revealed that a likely density gradient effect of type-1 radar echoes for the wavelength dependence of phase velocity would appear at a larger radar elevation angle while short wavelength waves would approach isotropic speeds close to ion acoustic speed. This interesting result is primarily attributed to the spectral features of 2D modes.