  A tunable microwave scattering device is presented which allows the
controlled variation of Fano line shape parameters in transmission through
quantum billiards. We observe a non-monotonic evolution of resonance parameters
that is explained in terms of interacting resonances. The dissipation of
radiation in the cavity walls leads to decoherence and thus to a modification
of the Fano profile. We show that the imaginary part of the complex Fano
q-parameter allows to determine the absorption constant of the cavity. Our
theoretical results demonstrate further that the two decohering mechanisms,
dephasing and dissipation, are equivalent in terms of their effect on the
evolution of Fano resonance lineshapes.
