In this work, the Method of Finite Element is applied in the electromagnetic analysis and project of coaxial horns employed as feed in omnidirectional reflector antennas. These feeds are usually excited by standard commercial connectors that require an adaptor to connect to the coaxial horn. This adaptor is usually composed of non-uniform sections of coaxial wave guide, where dielectric rings are used to give the mechanical strength to the structure. The adequate dimensioning of these coaxial sections leads to a compromise between the minimization of return loss, the required frequency operation band, and the necessary mechanical rigidity of the structure. Due to the non-uniformities, numerical methods are required to predict the electromagnetic behavior. In this work, an algorithm based on Method of Finite Elements is used to evaluate the performance of the sections that composed the feed chain. The performance of different types of horn sections and dimensions are explored by using the algorithm. To validate the results obtained by using Method of Finite Elements they are compared with those obtained by employing Method of Mode Matching.