  A theoretical explanation is given to account for the unexpected observation
that L- and S-band Nb superconducting cavities were found to have lower Q and
lower magnetic breakdown field than those of the higher X-band frequencies.
Both effects can be related to the trapping of magnetic flux in the cavity
walls. The frequency dependence arises from the frequency dependence of the
resistivity of oscillating fluxoids. Calculations based on this model are in
agreement with experimental observations.
