We report on microwave measurements of the surface impedanceZs=Rs+iXsofNb3Snsuperconducting films deposited by planar magnetron sputtering on sapphire substrates. The samples show theA15structure with maximumTc(R=0)of 17.4 K and residual resistivity ratios up to &bgr;≈3. The measurements are performed by a microstrip resonator technique using a meander line geometry. The power dependence ofZsis studied at different temperatures and frequencies in the range of 1–10 GHz using the fundamental and higher order modes of the resonator. The effect of a dc magnetic field applied with different orientations is also studied. The data are analyzed in the context of a simple model able to describe most of the reported loss mechanisms. ©1997 American Institute of Physics.