AbstractA model is suggested which accounts for (i) the observed shape and angular variation of compact radio sources (especially the apparent superrelativistic velocities and the absence of contracting sources), (ii) the flux variation associated with the angular variation, and (iii) all the known cases of apparent occurence of surface brightness exceeding the theoretical upper limit provided by the inverse COMPTONeffect, preserving the usual premises: cosmological origin of the redshift and incoherent synchrotron radiation of electrons. The model consists of a plasma ring expanding with moderate relativistic velocity.It provides two possibilities for estimating the distance of the sources by radio data: from the time dependence of the angular expansion and from the angular diameter and the shape of flux variation. In the context of cosmology this distances are „angular distances”︁ and therefore, if the redshift is also known, the HUBBLEconstant or the acceleration parameterq0may be obtained.The second method is applied to BL Lac yielding approximately 6 Mpc. So the underlying galaxy would be a dwarf system ofM≈︂–13. The active nucleus ofM≈︂ – 16 is rather below the normal quasars. This seems very satisfactory in view of the short time scale of variations in BL Lac compar