Input admittance of a circular waveguide aperture loaded with an iris

An expression for the input admittance of a circular aperture in an infinite groundplane, excited by a circular waveguide having diameter different from that of the aperture, is derived by using the principle of conservation of the flux of reaction. Assuming a TE11 mode field variation at the aperture, contributions of the TE and TM plane-wave components to the complex radiated power, and hence to the aperture admittance, are determined. A method for obtaining one of the integrals appearing in the expression for aperture admittance and another one in the form of convergent series is presented. The computation of input admittance is performed for the aperture diameter range 0¿2b/¿<0.97. A method of quantitative estimation of the effect of higher-order modes excited at the aperture is also presented.