In some examples, a device includes a waveguide transition section comprising a first mode suppressor, an attenuation section coupled to the first waveguide transition section via a first adjustable rotation joint, wherein the attenuation section is operable to attenuate the electromagnetic signal, and a first quarter-wave plate section coupled to the attenuation section, wherein the first quarter-wave plate section is operable to introduce a first differential phase shift between a first mode of the electromagnetic signal and a second mode of the electromagnetic signal. The device also includes a second quarter-wave plate section coupled to the first quarter-wave plate section via a second adjustable rotation joint, wherein the second quarter-wave plate section is operable to introduce a second differential phase shift between the second mode of the electromagnetic signal and the first mode of the electromagnetic signal.