  We have carried out non-linear microwave (8 GHz) surface impedance
measurements of three YBaCuO thin films in dc magnetic fields $H_{dc}$
(parallel to c axis) up to 12 mT using a coplanar resonator technique. In zero
dc field the three films, deposited by the same method, show a spread of
low-power residual surface resistance, $R_{res}$ and penetration depth,
$\lambda$ (T=15 K) within a factor of 1.9. However, they exhibit dramatically
different microwave field, $H_{rf}$ dependences of the surface resistance,
$R_s$, but universal $X_s(H_{rf})$ dependence. Application of a dc field was
found to affect not only absolute values of $R_s$ and $X_s$, but the functional
dependences $R_s(H_{rf})$ and $X_s(H_{rf})$ as well. For some of the samples
the dc field was found to decrease $R_s$ below its zero-field low-power value.
