Evolution of spectral indexes α_3.0−1.4 (top), α_1400−325 (middle), and α_325−150 (bottom) as a function of redshift for LOFAR radio-undetected quasars (RUQs). The purple circles represent the median RUQs; green circles are the RUQs detected at both frequencies; yellow or red circles indicated RUQs that are undetected in the upper or lower frequency, respectively; and blue circles are RUQs undetected at both frequencies. The horizontal dashed line indicates the canonical value of α = −0.7.