Left: comparison of spectral indices measured between 150 MHz and 1.4 GHz and 1.4 and 5 GHz. The double red arrows indicate objects only detected at 1.4 GHz, the blue arrows indicate those detected at 1.4 and 5 GHz, but not at 150 MHz, the black arrows indicate those detected at 150 MHz and 1.4 GHz, but not at 5 GHz. The solid lines separate flat (α <  0.5) and steep sources. Sources located above the dotted line (marking an equal value of the two spectral indices) are objects with a convex spectrum. The green cross on the upper left corner represents the maximum error, corresponding to the sources with flux density equal to 5 × σ in both TGSS and GB6. The cyan broken lines reproduce the typical spectral shape in different regions of the diagram. Right: same as left panel but limited to 19 sources with F_1.4 >  50 mJy.