  We study free-free absorption of radio emission by winds of massive stars. We
derive formulae for the optical depth through the wind measured from a point of
emission along a jet, taking into account Compton and photoionization heating
and Compton, recombination, line and advection cooling.
  We apply the developed formalism to radio monitoring data for Cyg X-1, which
allows us to obtain strong constraints on the structure of its inner jet. With
the data at 15 GHz, and taking into account an anisotropy of the stellar wind
in Cyg X-1, we estimate the location of the peak of that emission along the jet
at about one orbital separation, i.e., $\sim 3\times 10^{12}$ cm. Given a
previous determination of the turnover frequency in Cyg X-1, this implies the
location of the base of the jet at $\sim 10^3$ gravitational radii. We also
obtain corresponding results at 8.3 GHz and 2.25 GHz, which roughly follow the
standard conical partially self-absorbed jet model. Furthermore, we find that
the level of the orbital modulation depends on the radio flux, with the
modulation being substantially stronger when the radio flux is lower. This is
explained by the height of the radio emission along the jet decreasing with the
decreasing radio flux, as predicted by jet models. Based on the finding of the
flux-dependent orbital modulation, we are able to estimate a range of the
possible changes of the form of the radio/X-ray correlation in Cyg X-1 due to
free-free absorption. We also derive predictions for the orbital modulation and
flux attenuation at frequencies beyond the 2.25--15 GHz range.
