Radio spectrum of PMN J1603–4904 including archival data points from the ATCA, VLBI (Müller et al. 2014), MGPS (Murphy et al. 2007), the ALMA calibrator survey, TGSS (Intema et al. 2016), and GMRT (this work). The low-resolution data (i.e., except VLBI core fluxes) can be well described by a FFA (black) or SSA (gray) spectrum with a spectral turnover at ~400 MHz (observed frame). The highest measurement at 343.5 GHz by ALMA could either be due to a spectral break around 100 GHz or emission of the inner jet indicated by the power-law spectrum defined by the VLBI core (see Sect. 3).