  We present high frequency measurements of the vortex dynamics of a
FeSe$_x$Te$_{1-x}$ ($x=0.5$) thin film grown on a CaF$_2$ substrate and with a
critical temperature $T_c\simeq18\;$K, performed by means of a dual frequency
dielectric resonator at 16.4 GHz and 26.6 GHz. We extract and discuss various
important vortex parameters related to the pinning properties of the sample,
such as the characteristic frequency $\nu_c$, the pinning constant $k_p$ and
the pinning barrier height $U$ relevant for creep phenomena. We find that the
vortex system is in the single-vortex regime, and that pinning attains
relatively high values in terms of $k_p$, indicating significant pinning at the
high frequencies here studied. The pinning barrier energy $U$ is quite small
and exhibits a non-monotonous temperature dependence with a maximum near 12 K.
This result is discussed in terms of core pinning of small portion of vortices
of size $\propto\xi^3$ jumping out of the pinning wells over very small
distances, a process which is favoured in the high frequency, short ranged
vortex oscillations here explored.
