  We experimentally study the widths of resonances in a two-dimensional
microwave cavity at room temperature. By developing a model for the coupling
antennas, we are able to discriminate their contribution from those of ohmic
losses to the broadening of resonances. Concerning ohmic losses, we
experimentally put to evidence two mechanisms: damping along propagation and
absorption at the contour, the latter being responsible for variations of
widths from mode to mode due to its dependence on the spatial distribution of
the field at the contour. A theory, based on an S-matrix formalism, is given
for these variations. It is successfully validated through measurements of
several hundreds of resonances in a rectangular cavity.
