  We study the appearance of directed current in tunnel junctions, quantum
ratchet effect, in the presence of an external ac field f(t). The current is
established in a one-dimensional discrete inhomogeneous "tight-binding model".
By making use of a symmetry analysis we predict the right choice of f(t) and
obtain the directed current as a difference between electron transmission
coefficients in opposite directions, $\Delta T = T^{LR}-T^{RL}$. Numerical
simulations confirm the predictions of the symmetry analysis and moreover, show
that the directed current can be drastically increased by a proper choice of
frequency and amplitudes of the ac field f(t).
