Context :

Aims : We study the radio emission mechanism of the FR-I AGN NGC 5128 ( Centaurus A )

Methods : We determine the centimeter and millimeter continuum spectrum of the whole Centaurus A radio source and measure at frequencies between 86 GHz ( 3.5 mm ) and 345 GHz ( 0.85 mm ) the continuum emission from the active radio galaxy nucleus at various times between 1989 and 2005 .

Results : The integral radio source spectrum becomes steeper at frequencies above 5 GHz , where the spectral index changes from \alpha _ { low } = -0.70 to \alpha _ { high } = -0.82 .
The SW outer lobe has a steeper spectrum than the NE middle and outer lobes ( \alpha = -1.0 vs -0.6 ) .
Millimeter emission from the core of Centaurus A is variable , a variability that correlates appreciably better with the 20-200 keV X-ray variability than with 2 - 10 keV variability .

Conclusions : In its quiescent state , the core has a spectral index \alpha = -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 .