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A 1-18 GHz parabolic reflector horn antenna system featuring a broadband double ridged primary horn with a coaxial feed line is investigated. For the ridged horn antenna it is found that the radiation pattern, contrary to common belief, does not maintain a single main lobe in the direction of the horn axis over the whole frequency range. Instead, at frequencies above 12 GHz the main lobe in the radiation pattern starts to split into four lobes pointing in off-axis directions with a dip of up to 6 dB between them along the center axis. To investigate this phenomenon in detail, a combined method of moments and physical optics approach has been adopted to simulate the complete antenna system.