Applying the frequency-switching base equation, Eq. (36), one would expect the spectral-line ghosts to have half the amplitude of the original line. However, in practice one is usually unable to determine – required for Eqs. (34) and (35) – but only . As a consequence the inferred calibration is wrong at frequencies where the spectral-line ghosts appear. This does not only affect the amplitude of the ghosts, but even their shape gets distorted. To show the effect of this, we use a simple model in which different source intensities (T_sou = 5,10,20,50,100   K) were added to a hypothetical system temperature, T_sys = 100   K (top panel). The bandpass was assumed to be flat. For visualisation purposes, the spectral lines are triangular-shaped. Increasing the input source temperature with respect to T_sys decreases the line ratios of T_ghost to T_sou after applying frequency shifting (middle panel). In addition the distortion becomes more and more pronounced. Finally, in the bottom panel the result of a simple shift-flip-sign-averaging (so-called folding; see also Eq. (37)) is presented. The legend contains the reconstructed peak temperatures, which are systematically deficient for larger ratios of T_sou/T_sys. This folding algorithm should clearly be avoided for cases where T_sou3T_sys.