Examples of FRI (top two) and FRII (third and fourth) radio sources where the extended spectral index can be studied. See the text for details. From left to right, we plot the LBA, HBA, and Apertif images, the spectral indices LBA-HBA (the LBS contours are superposed), and HBA-Apertif (the Apertif contours are superposed). The contour levels are 3, 10, and 150 σ, where σ is the local noise level of LBA (left) and Apertif image (right; see text).