  We present the results from the first quasi-simultaneous multi-frequency
(2.3, 5.0, 8.4 and 15 GHz) Very Long Baseline Interferometry (VLBI)
observations of a compact steep spectrum (CSS) superluminal source 3C138. For
the first time, the spectral distribution of the components within its central
10 milli-arcsecond (mas) region was obtained. This enables us to identify the
component at the western end as the location of the nuclear activity on the
assumption that the central engine is associated with one of the detected
components. The possibility that none of these visible components is the true
core is also discussed. The new measurements further clarify the superluminal
motions of its inner jet components. The multi-frequency data reveal a convex
spectrum in one jet component, inferring the existence of free-free absorption
by the ambient dense plasma.
