  We present measurements of the magnetic field dependent microwave surface
resistance in laser-ablated YBa$_2$Cu$_3$O$_{7-\delta}$ films on SrTiO$_3$
substrates. BaZrO$_3$ crystallites were included in the films using composite
targets containing BaZrO$_3$ inclusions with mean grain size smaller than 1
$\mu$m. X-ray diffraction showed single epitaxial relationship between
BaZrO$_3$ and YBa$_2$Cu$_3$O$_{7-\delta}$. The effective surface resistance was
measured at 47.7 GHz for 60$< T <$90 K and 0$< \mu_0H <$0.8 T. The magnetic
field had a very different effect on pristine YBa$_2$Cu$_3$O$_{7-\delta}$ and
YBa$_2$Cu$_3$O$_{7-\delta}$/BaZrO$_3$, while for $\mu_0H=$0 only a reduction of
$T_c$ in the YBa$_2$Cu$_3$O$_{7-\delta}$/BaZrO$_3$ film was observed,
consistent with dc measurements. At low enough $T$, in moderate fields
YBa$_2$Cu$_3$O$_{7-\delta}$/BaZrO$_3$ exhibited an intrinsic thin film
resistance lower than the pure film. The results clearly indicate that
BaZrO$_3$ inclusions determine a strong reduction of the field-dependent
surface resistance. From the analysis of the data in the framework of simple
models for the microwave surface impedance in the mixed state we argue that
BaZrO$_3$ inclusions determine very steep pinning potentials.
