Frequency dependence of the noise level (blue curve, left axis) and the corresponding fraction of RFI-masked pixels (red dashed curve, right axis). The bottom axis shows the observed frequency in MHz, while the top axis indicates the corresponding redshift of the HI line. The secondary y-axis illustrates the fraction of RFI-masked pixels over the same frequency range. The gray curve represents the expected rms increase induced by RFI flagging, which is estimated via Equation (6).