Horn antennas are widely used in communication systems. They are typically used as feeds for re ector antennas and, due to its robustness, they can also be mounted on the fuselage of airplanes and are used on-board satellites. Horn antennas also present great directivities, gains, and e ciencies. The main drawback of this antenna family is (as it happens with most microwave devices) that they are pretty complicated to simulate with full-wave electromagnetic methods. Most of the commercial tools at the market for simulating microwave devices (CST Microwave Studio, HFSS,…) use general numerical methods such as finite elements or finite diferences in time or frequency domain, whose main advantage is that they can tackle a wide range of problems. The main drawback of these general methods is that this generality makes the simulation ine cient, causing long computation times for some problems in comparison with other more analytical techniques. The goal of this work is to develop a software tool capable of analyzing, simulating and designing horn antennas e ciently based on a numerical method, called Mode-Matching, which is known to be very e cient for this type of problem. Using this software would not only lead to shorter simulation and design times but would also permit to design horn antennas using low performance computers like notebooks.