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This paper demonstrates main-reflector antenna-distortion compensation using a novel sub-reflectarray compensation technique, through extensive simulations and measurements on a symmetric dual-reflector antenna system at Ku band. A sub-reflectarray Cassegrain system was used for the study. The distortion was created by adding an annular metallic ring to the main refl ector's surface. This ring-type distortion created pattern degradation and performance deterioration of the dual-reflector system. Back-projection holography was used to study the effects of the distortion on the antenna's performance. The conjugate-fi eld-matching method was used to obtain phase excitations at the sub-refl ectarray element locations for distortion compensation. A prototype microstrip-patch-type sub-refl ectarray was designed and fabricated. Radiation-pattern measurements and holographic diagnostics demonstrated the compensating technique, and showed that the sub-refl ectarray drastically improved the antenna's performance.