Mosaic coverage shown in equatorial coordinates. Radii of the circles correspond to the largest primary beam FWHM of the LoTSS survey (4.75°). Red, orange, blue, and grey circles mark Faraday space resolution of 1.44, 1.23, 1.20, and 1.16 rad m⁻², respectively (see Table 2). Areas previously studied by different authors are outlined in black (see Table 1 for more information). Symbols in the legend mark fields whose noise and spectra are shown in Figs. 1 and 3, where they are drawn with a corresponding colour and marked with numbers 1 through 4. Empty circles outlined in grey mark bad quality or missing observations.