  In this work, we study the realization of realistic polarization transformers
formed by long metallic helices. To this end, we propose a new homogenization
model to characterize the propagation of electromagnetic waves in medium formed
by infinitely long helices. We derive approximate analytical expressions for
the effective permittivity, effective permeability and the magnetoelectric
tensor of the composite material, taking into account the effects of spatial
dispersion. We apply the new homogenization model to characterize novel
linear-to-circular polarization transformers. Our results show that the
metamaterial screen may be designed in such a way that an incoming linearly
polarized wave may be transformed into a circularly polarized transmitted wave,
and that the transmission efficiency of such polarization transformer may be as
high as 95%. The theoretical results are supported by full wave simulations.
