Example FDF spectra from our ASKAP (943 MHz, top) and MeerKAT (2.41 GHz, bottom) polarisation cubes. The ‘on-source’ spectrum was measured within a 20 arcsec radius of the peak of diffuse polarised emission (RA, Dec) = (31.8849062° ,2.1790544° ); the ‘off-source’ spectrum within 20 arcsec of HCG15-D, the brightest continuum source embedded in the diffuse emission. The horizontal line indicates the 2σ_P level, where σ_P = 28.6 μJy beam⁻¹ for ASKAP and 2.96 μJy beam⁻¹ for MeerKAT.