Normalised gain-spectra of the DI-calibration for frequency range 134–147 MHz, observation L254871. The one-step unregularised calibration (‘LOFAR 2020’, blue lines) is compared with the two-steps calibration (‘This work’, red lines). The delay corresponding to reflections in the 115-metre cables of LOFAR is denoted by a grey line, where a spectral feature is also observed, as expected.