  A phenomenological model is proposed which describes the effect of both dc
and rf magnetic fields, $H$, on the microwave surface impedance, Z_s=R_s+jX_s,
of a superconductor containing weak link. Two cases are considered; a weak link
between two grains, shunted by another grain, and a non-shunted weak link. In
both cases the dependences of R_s and X_s on applied field H were found to be
anomalous. Under certain conditions, the values of Z_s(H) can fall below the
zero-field values. Comparison with experiment is performed, and very good
qualitative and (in some cases) quantitative agreement is found.
