A polarization converter may include an annular waveguide comprising an inner conductor having an outer surface and an outer conductor having an inner surface coaxial with the outer surface of the inner conductor. A plurality of loading structures may be disposed within the annular waveguide to form a plurality of regions within the annular waveguide including an alternating sequence of high phase shift regions and low phase shift regions along a direction of propagation of an electromagnetic wave. The plurality of loading structures may be configured to introduce a predetermined relative phase shift between orthogonally polarized first and second components of the electromagnetic wave for a predetermined operating frequency band. The plurality of loading structures may be further configured to suppress propagation of one or more higher order modes in the annular waveguide over the operating frequency band.