Radiation from axisymmetric waveguide fed horns

The finite element method (FEM) is used in conjunction with the method of moments (MoM) and the mode matching technique (MM) to calculate reflection coefficients and radiation patterns for axisymmetric waveguide fed horns. The coupling of FEM to the MoM, on one hand, and the coupling of the FEM to the MM, on the other, are performed by using boundary integrals. One advantage of this approach is that it allows for the presence of inhomogeneous materials to be included in the modelling domain as this poses no special problems for the FEM. In this respect, this method differs from the work of Berthon and Bills [1989] who use only the MoM with a single waveguide mode serving as the excitation. After describing the basic theory, the method is applied to a horn antenna on the Cassini spacecraft. The two dimensional modelling domain for this horn clearly shows where the FEM/MoM and the FEM/MM boundary surfaces are located. Comparisons of measured and calculated far field radiation patterns and reflection coefficients are then shown. It is noted that this method generates a sparse, diagonally dominant, complex-symmetric system matrix which may be solved with standard library routines. Moreover, this FEM formulation has been shown to be free of spurious solutions.