Comparison between two radio sources with the same flux, but different spectral indices. The black triangles denote the 5σ flux density limits for previous all-sky shallow low- and high- frequency surveys (Hales et al. 1988; Becker et al. 1995; Condon et al. 1998; Rengelink et al. 1997; Cohen et al. 2007; Heald et al. 2015; Intema et al. 2017), while color bars indicate the 3 different tiers for LOFAR surveys using the LOFAR Low band antennas (LBA) and High band antennas (HBA), and the deepest high-frequency surveys currently published (Schinnerer et al. 2010; Miller et al. 2013; Smolčić et al. 2017a). Sources steeper than α = −2.1 will be detected at higher signifcance in the Tier-2/Tier-3 surveys than in deep high-frequency surveys, while sources flatter than α = −0.75 at detected at both low and high frequencies.