A polarizer for an antenna and antenna structures are disclosed. According to one aspect a polarizer includes a generally cylindrical sidewall portion extending between spaced apart end portions. At least one polarizing structure extends substantially continuously along an interior of the sidewall portion and extends radially inwardly relative to the sidewall portion according to a first radius in a longitudinal direction and a second radius in a transverse direction. The polarizer can be implemented as part of an antenna structure, which further can be integrated with a transition stage and/or a horn.