As a reactance matching network, diaphragms in a circular waveguide is useful. The equivalent circuit parameters of this diaphragm is calculated by using variational method.
For the incident field of TE_{11} mode, one may calculate reactances of the circular diaphragms by using variational method and computer.
When the opening of the diaphragm is large, the equivalent circuit parameters are obtained by representing the aperture field as a superposition four modes. When its opening is small, the reantances are obtained by assuming the aperture field as form of diverging at edge.
Both methodes approaches when the aperture size is about $\frac{1}{2}$ of the waveguide.
