  In a study of the radio emission mechanism of the FR-I AGN NGC 5128
(Centaurus A)}, we have determined the centimeter and millimeter continuum
spectrum of the whole Centaurus A radio source and measured the continuum
emission from the active galxy nucleus at various times between 1989 and 2005
at frequencies between 86 GHz (3.5 mm) and 345 GHz (0.85 mm). The integral Cen
A spectrum becomes steeper at frequencies above 5 GHz, where the spectral index
changes from -0.70 to -0.82. Millimeter emission from the core of Centaurus A
is variable, and correlates appreciably better with the 20-200 keV than the 2 -
10 keV X-ray variability. In its quiescent state, the core spectral index is
-0.3, which steepens when the core brightens. The variability appears to be
mostly associated with the inner nuclear jet components that have been detected
in VLBI measurements. The densest nuclear components are optically thick below
45-80 GHz.
