  An ensemble of particles in thermal equilibrium at temperature $T$, modeled
by Nos\`e-Hoover dynamics, moves on a triangular lattice of oriented semi-disk
elastic scatterers. Despite the scatterer asymmetry a directed transport is
clearly ruled out by the second law of thermodynamics. Introduction of a
polarized zero mean monochromatic field creates a directed stationary flow with
nontrivial dependence on temperature and field parameters. We give a
theoretical estimate of directed current induced by a microwave field in an
antidot superlattice in semiconductor heterostructures.
