The AC linear response of YBaCuO films prepared by RF magnetron sputtering to a small oscillating field was studied as a function of temperature and transverse magnetic field. The structures observed are quite similar to those found in artificially prepared two-dimensional periodic junction arrays where the transition from a phase-ordered to a phase-disordered state at Tc is attributed to the vortex unbinding mechanism predicted by the Kosterlitz-Thouless theory for phase transitions in two dimensions. The magnetic field data suggest that the typical cell size of the YBCO array is a few μm, on the order of the grain size. This suggests that the ceramic films can be modeled as two-dimensional networks of Josephson tunnel junctions, the junctions being located at the grain boundaries.