A boundary element code is used to study the electromagnetic field scattered by a helix at microwave frequencies. The far-field forward and backward scattering amplitudes are computed for an incident linearly polarized wave with various polarization directions, and a number of orientations of the helix. These computations allow us to study the magnitude of the field scattered in the forward and backward directions as a function of frequency and orientation. They are also the starting point for computing the effective wavenumbers of left and right circularly polarized waves in a chiral composite made of a dispersion of helices in a host medium. Several methods to compute the effective impedance of chiral composites in order to obtain all three constitutive parameters of such media are proposed. Numerical results are given and compared with existing experimental and theoretical data.