  Compact radio cores associated with relativistic jets are often observed in
both active galactic nuclei and X-ray binaries. Their radiative properties
follow some general scaling laws which primarily depend on their masses and
accretion rates. However, it has been suggested that the black hole spin can
also strongly influence the power and radio flux of these. Here, we attempt to
estimate the dependency of the radio luminosity of steady jets launched by
accretion disks on black hole mass, accretion rate and spin using numerical
simulations. We make use of 3D GRMHD simulations of accretion disks around
low-luminosity black holes in which the jet radio emission is produced by the
jet sheath. We find that the radio flux increases roughly by a factor of 6 as
the back hole spin increases from a~0 to a=0.98. This is comparable to the
increase in accretion power with spin, meaning that the ratio between radio jet
and accretion power is hardly changing. Although our jet spine power scales as
expected for the Blandford-Znajek process, the dependency of jet radio
luminosity on the black hole spin is somewhat weaker. Also weakly rotating
black holes can produce visible radio jets. The overall scaling of the radio
emission with black hole mass and accretion rate is consistent with the
scale-invariant analytical models used to explain the fundamental plane of
black hole activity. Spin does not introduce a significant scatter in this
model. The jet-sheath model can describe well resolved accreting systems, such
as SgrA* and M87, as well as the general scaling behavior of low-luminosity
black holes. Hence the model should be applicable to a wide range of radio jets
in sub-Eddington black holes. The black hole spin has an effect on the
production of visible radio jet, but it may not be the main driver to produce
visible radio jets. An extension of our findings to powerful quasars remains
speculative.
