The far-field radiation patterns of conical horns of arbitrary flare angles excited in the TE11 mode are obtained employing the geometric theory of diffraction (GTD) based on the theory of Kouyoumjian and Pathak  and the slope diffraction technique . The analysis presented enables one to predict accurately radiation patterns over the main beam, near and far sidelobes, and the backlobe of the horn. Validity of the analysis is established by satisfactory agreement between the calculated and measured patterns of an experimental conical horn. The radiation patterns of wide-flare corrugated conical horns excited in the HE11 mode of operation have also been calculated over the main beam, which contains most of the radiated energy (up to −40 dB with respect to boresight field), employing slope diffraction technique, and a good agreement is noticed between the calculated and measured radiation patterns. Copyright © 1978 by The Institute of Electrical and Electronics Engineers, Inc.