  We extend previous works by considering two additional radio frequencies (K
band and X/Ka band) with the aim to study the frequency dependence of the
source positions and its potential connection with the physical properties of
the underlying AGN. We compared the absolute source positions measured at four
different wavelengths, that is, the optical position from the Gaia Early Data
Release 3 (EDR3) and the radio positions at the dual S/X, X/Ka combinations and
at K band, as available from the third realization of the International
Celestial Reference Frame (ICRF3), for 512 common sources. We first aligned the
three ICRF3 individual catalogs onto the Gaia EDR3 frame and compare the
optical-to-radio offsets before and after the alignment. Then we studied the
correlation of optical-to-radio offsets with the observing (radio) frequency,
source morphology, magnitude, redshift, and source type. The deviation among
optical-to-radio offsets determined in the different radio bands is less than
0.5 mas, but there is statistical evidence that the optical-to-radio offset is
smaller at K band compared to S/X band for sources showing extended structures.
The optical-to-radio offset was found to statistically correlate with the
structure index. Large optical-to-radio offsets appear to favor faint sources
but are well explained by positional uncertainty, which is also larger for
these sources. We did not detect any statistically significant correlation
between the optical-to-radio offset and the redshift. The radio source
structure might also be a major cause for the radio-to-optical offset. For the
alignment of with the Gaia celestial reference frame, the S/X band frame
remains the preferred choice at present.
