A classical radioastronomy receiver is fed with a corrugated horn and an independent lens,
both placed in a cryostat to lower the noise temperature. The beam is focused and directed using a
combination of elliptical and plane mirrors. This paper proposes modifying the initial feeding system
by placing the lens onto the horn aperture, thereby allowing a size reduction of the horn and lens, and a
simplification of their mechanical design. The profiled lens is shaped to correct the phase error on the
horn aperture. A quasi-optical model of the horn-plus-lens system has been developed using a Beam
Mode Expansion (BME). Results using both a hyperbolic-planar lens and a spherical-elliptical lens,
as well as results obtained by using Geometrical Optics (GO) with a Kirchoff–Huygens integration
to get the far-field pattern, have been compared with measurements. As a direct application, a full
focusing system for the new 40-m radiotelescope at the “Centro Astron´omico de Yebes” is presented
for the 22, 30 and 45 GHz bands. This paper has developed a QO model for a corrugated conical horn
with a phase-correcting lens
