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Over-the-horizon (OTH) HF radars work in a heavily congested HF band. It is quite difficult to find broad clear frequency bands for system requirements. The randomly discontinuous spectra (RDS) signal is employed to combat spectrum congestion since it can evade the external interferences in the frequency domain. However, the spectra discontinuity of the signal gives rise to high range sidelobes when matching the reflected echo, which is much more difficult for target detection. So it is indispensable to investigate the technique for sidelobes suppression of the range profile when an RDS signal is utilized. In this paper, we introduce a new signal processing technique, that is radically different from the conventional technique, to lower range sidelobes based on suppressing the self-clutter of the radar range ambiguity function (AF) by mismatch filtering. Simulation results show that the peak sidelobe level can be reduced to -30 dB while the frequency bands span up to 400 kHz.