Design of Multimode Horn Feed for Parallel Plate Waveguide-Based Offset Dual-Reflector Antenna

This paper presents the design of a W-band multimode horn feed (MHF) to illuminate an offset dual-reflector system in a parallel-plate waveguide (PPW), aiming to improve the antenna efficiency by achieving nearly uniform aperture illumination. While conventional horns typically achieve <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\sim 80 \%$</tex> aperture efficiency in classical reflector systems, an open question is how closely multimode horn feeds can approach the theoretical efficiency limit in PPW-based architectures. The proposed MHF incorporates a horizontal stepped waveguide structure to excite higher-order modes, each with a specific complex coefficient, thereby producing a sub-reflector illumination pattern that closely approximates the ideal secant profile. The MHF design has been explored for both single-polarized and dual-polarized configurations, achieving a taper efficiency of 97% for the reflector system in PPW. The design trade-offs for achieving high taper efficiency and minimal spillover loss, while addressing challenges such as multiple sidewall reflections and phase aberrations in closed metallic structures, are also discussed.