Eigenoscillations of plane junctions and total transmission through small holes

Regimes of resonance transmission through a small below-cutoff hole in a metallic waveguide iris are studied. The resonances are caused by the excitations of specific eigenoscillations of the iris as an open waveguide cavity. Such eigenoscillations are found for irises in various waveguides. Common origin of the eigenoscillations that is peculiar to a plane junction of the hollow waveguides is studied.