In this contribution the mode matching technique is applied to compute the absorption characteristics of a twodimensional array of rectangular waveguides. This analysis is motivated by a proposal of a broadband absorber for TESLA. Outside the waveguide array the so-called Rayleigh expansion is used which represents the electromagnetic field in terms of space harmonics whereas the complete modal spectrum of eigenmodes is taken into account inside the waveguides. In the case of normal incidence the validity of the presented method is confirmed by MAFIA computations. The absorption characteristics for various angles of incidence are calculated for a broad frequency range. The accuracy of the results is checked by a study of convergence. It is shown that the Rayleigh expansion has to be modified if the waveguide array is excited by an ultra-relativistic beam instead of an incoming plane wave. Numerical results for the beam parameters are presented for an array of parallel-plate waveguides and compared with those obtained by other methods
