One drawback of horn antennas is the difficulty in performing the required computations for their analysis and design. With the actual development of CAD tools these problems have been reduced but they are still far from being completely solved. Most commercial CAD tools employ general-purpose numerical techniques (like Finite Differences Method and Finite Elements Method) since they allow using the same software to simulate a wide range of devices. The disadvantage of these methods lies on the fact that this generality comes at a cost, the loss of efficiency. On the other hand, quasi-analytical techniques are usually more efficient but they can only work with a narrow spectrum of problems. The main objective of this work is to develop a software tool capable of analysing, simulating and designing horn antennas efficiently. To accomplish this, a quasi-analytical method called Mode-Matching will be used.