We measure the microwave surface impedance of YBa2Cu3O7-delta thin fil
ms in the mixed state, for films produced using PLD and electron beam
co-evaporation techniques. Measurements were made as a function of fie
ld (0-4 T) and temperature (4 K - T-C) at 8 GHz and 16 GHz. From these
measurements we extract the flux line restoring force constant (kappa
(p)), viscosity (eta), and pinning potential well depth (U). In a magn
etic field, deviation from an R(s) proportional to omega(2) indicates
a small but significant thermal activation of the flux lines. A field
dependent kappa(p) is observed for all samples, where kappa(p) proport
ional to B--alpha, and alpha>0. Furthermore a variation in the magnitu
de and field dependence of kappa(p) is observed between samples, where
the largest values of alpha (similar to 0.4) are observed in the samp
les with the smallest values of kappa(p).