Reduced Coupling Aperture of Microstrip Stubs Provides New Aspects in Stub Filter Design

Generally microstrip stubs are coupled galvanically over the whole cross-section of the adjoining main lines. Reducing the coupling aperture two effects can be observed being superimposed: Smaller coupling apertures lead to higher skirt selectivity, increase, however, the excitation of higher-order modes in the main lines. The resulting transmission characteristics seem to provide new applications in stub filter design.