  The fidelity decay in a microwave billiard is considered, where the coupling
to an attached antenna is varied. The resulting quantity, coupling fidelity, is
experimentally studied for three different terminators of the varied antenna: a
hard wall reflection, an open wall reflection, and a 50 Ohm load, corresponding
to a totally open channel. The model description in terms of an effective
Hamiltonian with a complex coupling constant is given. Quantitative agreement
is found with the theory obtained from a modified VWZ approach [Verbaarschot et
al, Phys. Rep. 129, 367 (1985)].
