Tri-Mode Horn Feeds Revisited: Cross-Pol Reduction in Compact Offset Reflector Antennas

The design of a tri-mode horn feed with novel structure is revisited in this communication. This design allows a significant reduction in the cross-pol level in the far-field patterns of linearly polarized offset parabolic reflector antennas with low F/D. This is, in particular, advantageous in spaceborne applications where the space and weight is limited. It is demonstrated that TE₂₁ mode in conical horns possesses a similar aperture field distribution as the focal plane field of an offset reflector antenna and can reduce the cross-pol level if the mode coefficients are optimized. The complex mode coefficients are optimized for a particular application of a 4.5 m offset reflector antenna operating at 10 GHz with F/D = 0.4. It is shown that the cross-pol level is reduced dramatically (~28 dB over 3% of bandwidth) when the reflector is fed by the tri-mode horn with optimum coefficients. The results suggest that this design could be very useful for future narrow band spaceborne remote sensing applications.