  This study proposes that a metallic helix array can operate as a
highly-transparent broadband wave plate in propagation directions perpendicular
to the axis of helices. The functionality arises from a special property of the
helix array, namely that the eigenstates of elliptically right-handed and
left-handed polarization are dominated by Bragg scattering and local resonance
respectively, and can be modulated separately with nearly fixed difference
between their wavevectors in a wide frequency range. The wave plate
functionality is theoretically and experimentally demonstrated by the
transformation of polarized states in a wide frequency range.
