GTD analysis of the principal plane radiation patterns of open-ended circular’ cylindrical waveguide horns excited in the TE11mode is presented, based on the theory of Kouyoumjian et al. – and the slope diffraction formulation . The study enables one to predict accurately the E-and H-plane patterns over the near and far sidelobes and backlobes of the horn, and a good agreement is noticed between the calculated and measured radiation patterns obtained with an experimental circular waveguide horn excited in the dominant mode. Copyright © 1979 by The Institute of Electrical and Electronics Engineers. Inc.