  A unified description is provided for the 21cm signatures arising from
minihaloes against a bright background radio source and against the Cosmic
Microwave Background (CMB), within the context of a dynamical collapsing
cosmological spherical halo model. The equivalent width distribution of the
resulting 21cm forest is computed for LCDM cosmologies, along with the
brightness temperature differential relative to the CMB. The effects of an
ambient Ly-alpha radiation field and heating of the IGM on the signatures are
included. It is shown that the dynamical effects of heating substantially
suppress absorption features in the 21cm forest with observed equivalent widths
exceeding 0.15 kHz. It is demonstrated how measurements of excess fluctuations
beyond detector noise could make a statistical detection of the weaker
absorption features against a bright background radio source. It is also shown
that a Ly-alpha radiation field only a few percent the thermalisation rate is
sufficient to render the minihalo signal against the CMB negligible compared
with the signal from the diffuse IGM component. The 21cm signals are found to
be very sensitive to the amount of small scale power in the primordial density
fluctuation spectrum. The effects of gas cooling via radiative atomic and
molecular processes and of star formation on setting the maximum mass of the
minihaloes giving rise to a 21cm signal are included, with allowance made for
the suppression of molecular hydrogen formation by an ambient UV radiation
field.
