Study of the partially open cavity´s resonant character

Two methods are used to study the resonant character of a partially open cavity. They are the waveguide modal approach and the extended boundary integral method (BIM). Numerical experiments indicate that the far field contribution of the resonant modes in a cavity is zero. So no attention should be paid to these modes for far field calculation. Thus the difficulty of calculating cavities´ far field radiation only relies on numerical instability of matrix inversion. This can be solved by SVD+BIM for the reason that singular value decomposition is a powerful and stable algorithm for dealing with matrices with very large condition number.