Components to the 3C 273 spectral energy distribution. Data are from Perley & Meisenheimer (2017), except for the 151 MHz flux density of the halo, which is from Punsly & Kharb (2016). The flux density of the jet includes all knots. The halo refers to the diffuse cocoon of radio emission encompassing the jet (A, B, and R of Fig. 9). The radio spectral indices of the core, jet, and halo are 0.30 ± 0.07, − 0.81 ± 0.03, and − 0.79 ± 0.11, respectively. The spectral indices of the jet and the halo are consistent with optically thin synchrotron emission, as expected. The LOFAR HBA bandwidth is shown by the shaded region.