Part of a transmission spectrum of the empty microwave sector billiard. The positions of the resonances yield the eigenfrequencies of the microwave billiard, that is, the eigenvalues of the corresponding quantum billiard. The zooms into the spectrum show two isolated resonances (left and right) and a pair of weakly overlapping ones (middle). The measurement of the electric field intensity distribution at either of the two eigenfrequencies of the latter yields a superposition of both distributions. Therefore, we only measured distributions at isolated resonances like those depicted in this figure.