  The microwave response of high quality niobium films in a perpendicular
static magnetic field has been investigated. The complex frequency shift was
measured up to the upper critical fields. The data have been analyzed by the
effective conductivity model for the type-II superconductors in the mixed
state. This model is found to yield consistent results for the coherence
lengths in high-kappa superconducting samples, and can be used with HTSC even
at temperatures much below T_c. It is shown that for samples with high values
of depinning frequency, one should measure both components of the complex
frequency shift in order to determine the flow resistivity. The thick Nb film
(160 nm) has low resistivity at 10 K, comparable to the best single crystals,
and low kappa value. In contrast, the thinnest (10 nm) film has kappa ~ 9.5 and
exhibits a high depinning frequency (~20 GHz). The upper critical field
determined from microwave measurements is related to the radius of
nonoverlaping vortices, and appears to be larger than the one determined by the
transition to the normal state.
