  We construct a supersymmetric field theory for the problem of a
two-dimensional electron gas in a random, static magnetic field. We find a new
term in the free energy, additional to those present in the conventional
unitary sigma-model, whose presence relies on the long-range nature of the
disorder correlations. Under a perturbative renormalization group analysis of
the free energy, the new term contributes to the scaling function at one-loop
order and leads to antilocalization.
