A new type of waveguide discontinuities-capacitive fin-septa windows are investigated by use of the resoance method combined with singular integral equation technique. Two ideal short circuiting planes are inserted at some distances from reference planes of the window so that the configuration forms a closed resonantor. Based on the resonant condition the equivalent parameters of the window are expressed in terms of the resonant distances between the reference planes and the shorting planes. The characteristic equation for the dista- nces with superior convergence is derived by using singular integral equation technique. As example

the equivalent parameters of a capacitive bilateral fin-septa window are computed. When the structure is degenerated into a capacitive metal window with zero thickness

the numerical results show good agreement with available data in the literatures.