A new designs of an ultrawideband (1–15 GHz) orthomode transducer and a dualpolarized quad-ridged horn antenna based on it are presented. In the operating frequency range VSWR of the orthomode transducer doesn't exceed 1.8 and the antenna's VSWR doesn't exceed 2.1. Isolation between two ports with orthogonal linear polarizations is higher than 40 dB and the antenna's gain varies from 5 to 14 dB.