A monopulse horn feed with a triple-radiation pattern (sum, elevation difference, and azimuthal difference) operating at 35 GHz is presented. The device consists of three main parts: a spline profiled horn, a mode converter that generates the modal field combinations that produce each radiation pattern, and a very compact single-layer monopulse comparator network. Unlike other state-of-the-art designs, the novel feed is based on a triple-radiation pattern horn with a mode converter excited by a comparator network implemented in a single level for improving the compactness and easing quasi-2-D manufacturing. The kernel of this network is a novel 180° H-plane waveguide coupler with in-line ports, which avoids any additional phase compensation in the routing waveguide sections used in other works. The design process of the Ka-band antenna feed is described, and the simulated results for the three radiation patterns and their associated reflection coefficients are presented. The prototyping process is outlined, and the obtained experimental results are compared with the simulated ones showing very good agreement and validating the design process.