Mode conversion in dielectric loaded horns

Abstract

The dielectric-cone-loaded horn creates a balanced hybrid mode by the inhomogeneously filled conical horn. Analytical models based on a single hybrid mode predicted a crosspolarization level which was greater than the measured result. The authors now show that good horn performance is caused by the presence of higher-order modes in the aperture. For a horn with a semiflare angle of 12 degrees , an aperture diameter of 154 mm, a dielectric with relative permittivity of 1.13, and air gap of 12 mm, the power at the aperture is made up of 77% HE/sub 11/ cylindrical mode, 16% HE/sub 12/ cylindrical mode and the remainder of the other higher order modes. This shows that a substantial amount of higher-order mode is present. To understand the mechanism for the mode conversion, the power in the first three cylindrical hybrid modes was computed and is shown as a function of radius for the horn. The mode conversion is partly due to the throat region and partly due to the inhomogeneous dielectric horn; the first component can be reduced by smoothly tapering the transition from the waveguide to the horn. >