Metallic helical metamaterials give rise to broadband and scalable chiro-optical effects orders of magnitude higher than found in nature. While arrays of gold helices have been suggested as compact circular polarizers, where a large difference of the diagonal elements of the Jones transmission matrix is desired, chiral metamaterials can also be designed to exhibit strong circular-polarization conversions. Here, a novel helical metamaterial design, exhibiting asymmetric, broadband circular-polarization conversion, is introduced. The metamaterial is composed of unit cells with a single helix that changes its handedness halfway along the helix axis. Based on numerical calculations, an intuitive model explaining the principle of operation is given. Furthermore, a novel fabrication approach employing STED-inspired direct laser writing in combination with electrochemical deposition of gold is presented. The experimental data show circular-polarization conversion of up to 75% for an unmatched bandwidth of one octave, in very good agreement with theory.