Modal propagation of electromagnetic waves in waveguides filled with chiral media, known as chirowaveguides, is studied. The waveguide walls are assumed to behave as constant impedance surfaces. This can model corrugated surfaces, dielectric coated metallic surfaces, as well as perfectly or imperfectly conducting walls. Conditions leading to modes with pure circular polarization are derived. Under these conditions, a chirowaveguide can act as a low crosspolar feed for reflector antennas.