S11 Calculation for a Coaxial-loaded-type Stepped Cut-off Circular Waveguide by the Moment Method

A formula for calculating the $\mathbf{S}_{\mathbf{11}}$ value of a stepped circular waveguide with a coaxial feed was formulated using the method of moments (MoM). The analytical model for the formulation of $\mathbf{S}_{\mathbf{11}}$ calculation was assumed with 1. no sample inserted (open) and 2. pure water inserted. $\mathbf{S}_{\mathbf{11}}$ for the analytical model was calculated at frequencies of 0.50, 1.5 and 3.0 GHz. The MoM-calculated $\mathbf{S}_{\mathbf{11}}$ values were compared with those calculated using the mode-matching technique (MMT) and the finite element method (FEM). The MoM values corresponded closely to the MMT values and the FEM values, even though the inner diameter of the circular cavity was slightly larger than the inner diameter of the outer conductor for the coaxial line. The formulation of the equation for the analytical model was thus validated.