  The field-induced variations of the microwave surface resistance are
investigated in two different samples of powdered MgB2. The experimental
results can be justified in the framework of the Coffey and Clem model with
fluxons moving in the flux-flow regime, provided that the anisotropy of the
upper critical field is taken into due account. Assuming the angular dependence
of the upper critical field expected from the anisotropic Ginzburg-Landau
theory, we determine the anisotropy factor by fitting the experimental data of
the field dependence of the microwave surface resistance. We show that the
anisotropy factor is constant in a range of temperatures of about 3 K below Tc
and it takes on different values for the two samples, even though they have
similar shape.

Superconductivity