A curved-aperture corrugated horn having very low cross-polar performance

The design and characteristics of an HE
11
hybrid mode corrugated horn that has a flared aperture of constant radius of curvature and the design factors relating to the corrugated waveguide mode converter required to feed the horn are described in detail. Provided the radius of curvature is greater than approximately 1.2 wavelengths at the design frequeney
f_{0}
, the cross-polar sidelobe intensity versus frequency characteristic of the horn is well defined and has a minimum which is below -50 dB near
f_{0}
. This type of horn is therefore suggested for use as a reference where a very low level of cross polarization is desired. Normalized curves are presented showing the cross polarization, the beamwidth, and the position of the phase center as functions of the horn parameters. The bandwidth achieved of 1.36:1 is limited not by the cross-polar performance, but by the rapid deterioration of the return loss near the lower edge of the band. The bandwidth, for a given cross-polar level, increases as the radius of curvature increases. The precautions necessary to achieve the low levels of cross polarization, and the fundamental principles in the design of the mode converter are discussed in detail.