DD1EG maybe an attenuator is needed to lower the noise level. A simpler solution could be to reduce the gain of the RTL SDR in order to increase the dynamic range. A typical SNR is about 20-25 dB.
The procedure that I use is to receive the CW beacon , starting the RF gain at minimium and as you increase the SNR and noise level increases. After one point the SNR does not increases but increases the noise only. I selcted the minimum gain that I archive this 'stable' SNR in order to have the best dynamic range as RTL is only 8 bits.