General Atomics supplies polarization rotators consisting of two 135° miter bends separated by a 90° miter bend. These can be used with variable beam splitters in ECE diagnostic transmission lines, for example.
This GA polarization rotator provides arbitrary rotation of linear polarization over a very wide range of frequencies. The polarization is rotated by twice the angle between the incident polarization and the axis of the rotator.
John Doane, GA-MFE Technologies