- Spectral properties of rf radiation from intrinsic Josephson junctions in high Tc Y‐Ba‐Cu‐O thin film have been measured in the frequency range up to 1.5 GHz. Narrow emission lines with the 3 dB bandwidth of the order of 20 MHz were detected indicating that Josephson clusters radiate coherently. Synchronization conditions are determined by dc current and external magnetic field bias. Frequency locking of radiation to external resonant circuit was also observed. Spectral line narrowing due to resonant lock was distinguished from the coherence‐induced narrowing by different tuning properties of the emission line. Noncoherent Josephson radiation manifests itself as a broadband background noise increase. A pronounced 1/f‐like tail sensitive to dc bias and magnetic field was observed in a low frequency part of the spectrum.