A radiometer developed at Budker Institute of Nuclear Physics (Siberian Branch, Russian Academy of Sciences) for diagnostics of the efficiency of the electron cyclotron plasma heating in a mirror magnetic trap (GDT) is discussed. The radiometer is based on the superheterodyne principle. The characteristics of the instrument such as the operating range, sensitivity, frequency and time resolution are adapted to the requirements of the experiments. The receiver is equipped with a system for protection against stray radiation from powerful gyrotrons that are used in the electron cyclotron heating experiments. The radiometric measurements of the plasma self-emission carried out at the GDT facility showed that the new information channel helps to interpret the processes of resonant microwave plasma heating more accurately.