  The corrugated horn is a high performance feed often used in radio
telescopes. There has been a growing demand for wideband optics and corrugated
horns in millimeter and submillimeter-wave receivers. It improves the
observation efficiency and allows us to observe important emission lines such
as CO in multiple excited states simultaneously. However, in the
millimeter/submillimeter band, it has been challenging to create a conical
corrugated horn with a fractional bandwidth of ~60% because the wavelength is
very short, making it difficult to make narrow corrugations. In this study, we
designed a conical corrugated horn with good return loss, low
cross-polarization, and symmetric beam pattern in the 210-375GHz band (56%
fractional bandwidth) by optimizing the dimensions of the corrugations. The
corrugated horn was installed on the Osaka 1.85-m mm-submm telescope with the
matched frequency-independent optics, and simultaneous observations of 12CO,
13CO, and C18O (J = 2-1, 3-2) were successfully made. In this paper, we
describe the new design of the corrugated horn and report the performance
evaluation results including the optics.
