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In this paper we investigate the radiation pattern of a complete 1-18 GHz antenna system featuring a 1.8 in parabolic reflector and a double ridged broadband feed horn with coaxial input using a combination of the method of moments (MoM) and a physical optics (PO) approach. It was found that in the lower frequency range the horn antenna system exhibits a single main lobe in the direction of the center horn axis as expected. However, contrary to common belief, at frequencies greater than 12 GHz, the main lobe starts to split into four main lobes pointing into off-center-axis directions with a broadside gain reduction. This effect was observed both theoretically and experimentally. Although this type of antenna has existed for over four decades, up to now there is no explanation for this unwanted behavior in the open literature. In the past, several electromagnetic simulations of ridged horn antennas were made, but not over such a broad operational frequency range and neither including the coaxial feed nor the complete parabolic reflector system. The purpose of this investigation is to close this gap and to contribute to a better understanding of broadband ridged horn antennas.