  Following a wave-mechanical treatment we calculate the drag force exerted by
an infinite homogeneous background of stars on a perturber as this makes its
way through the system. We recover Chandrasekhar's classical dynamical friction
(DF) law with a modified Coulomb logarithm. We take into account a range of
models that encompasses all plausible density distributions for satellite
galaxies by considering the DF exerted on a Plummer sphere and a perturber
having a Hernquist profile. It is shown that the shape of the perturber affects
only the exact form of the Coulomb logarithm. The latter converges on small
scales, because encounters of the test and field stars with impact parameters
less than the size of the massive perturber become inefficient. We confirm this
way earlier results based on the impulse approximation of small angle
scatterings.
