Reflection, Transmission, and Mode Conversion in a Corrugated Feed

Abstract

Microwave antennas are often required to carry signals simultaneously over a broad range of frequencies–e.g., the combined td-2 and th common carrier bands encompass a total frequency ratio of about 1.8 to 1 as do the combined 18- and 30-GHz bands. To achieve these bandwidths, an efficient broadband feed horn is required. The corrugated (hybrid-mode) horn is a leading candidate, but it is not immune to some cross-polarization coupling, input reflection, and pattern asymmetry. These problems are introduced mainly by two phenomena: variation of the dominant mode shape with frequency and mode conversion along the horn taper and at waveguide transitions at the horn input. Simple formulas for computing the magnitude of these phenomena and their effects on return loss and radiation patterns are given.