  We report results from a multiwavelength campaign to measure the simultaneous
spectrum of the super-massive black hole candidate Sgr A* in the Galactic
Center from cm to mm-wavelengths using the VLA, BIMA, the Nobeyama 45m, and the
IRAM 30m telescopes. The observations confirm that the previously detected
mm-excess is an intrinsic feature of the spectrum of Sgr A*. The excess can be
interpreted as due to the presence of an ultra-compact component of
relativistic plasma with a size of a few Schwarzschild radii near the black
hole. If so, Sgr A* might offer the unique possibility to image the putative
black hole against the background of this component with future mm-VLBI
experiments.
