  Magnetic impurities affect the spectrum of excitations of a superconductor
and thus influence its impedance. We concentrate on the dissipative part of the
surface impedance. We investigate its dependence on frequency, the density and
strength of magnetic impurities, and the density and temperature of
quasiparticles. Even a small concentration of weak magnetic impurities
significantly modifies the excitation spectrum in the vicinity of the BCS gap.
Therefore, we give special attention to the absorption threshold behavior at
zero temperature and to the low-frequency absorption by quasiparticles. The
discrete energy states introduced at low density of magnetic impurities may
serve as traps for nonequilibrium quasiparticles, reducing the absorption in
some range of low radiation frequencies.
