Modelling Few-moded Horns for Far-ir Space Applications

Few-moded or over-moded horns are now being proposed for far-IR space receiver systems, e.g. the High Frequency Instrument (HFI) on the ESA PLANCK Surveyor. In such systems individual waveguide modes can couple independently to an overmoded detector (such as a bolometer in an integrating cavity). We consider in detail the case of a cylindrically symmetric configuration consisting of a corrugated conical horn connected to waveguide filter also corrugated, such as is proposed for the HFI instrument. Such antenna feeds have the advantage of high coupling efficiency combined with low sidelobe level beam patterns. In the paper we present two alternative techniques for modelling few moded horn antennas. The first is based on a scattering matrix description of propagation in a non-uniform wave-guide structure, while the second approach uses hybrid mode solutions for a waveguide with corrugated walls. We also present computed and experimental data for an example prototype HFI horn.