Low‐field ac magnetic properties of high‐Tc superconductors are studied using a model of a granular system with weak links between grains. The existence of closed loops consisting of grains connected by small area Josephson contacts is assumed. The periodicity condition of the superconducting order parameter phase leads to the family of the energy branch corresponding to different phase ‘winding’ numbers. When the loop self‐inductance L is taken into account, the hysteresis occurs for the applied magnetic flux close to half‐integer values of flux quantum &Fgr;0. The width of the hysteresis range is proportional to LI0/N&Fgr;0, whereNis the number of weak links in the loop, Ic is the contact critical current. In this model the ac susceptibility is found, its imaginary part is due to the energy dissipation in the branch switching processes. The dependence of the susceptibility on the ac field amplitude, the second harmonic power of the magnetization vs dc magnetic field, temperature variation of the dissipative part of the susceptibility are calculated. Statistical properties of the medium are taken into account by an average in loop areas.