The Impact of Stray Magnetic Fields on the Performance of Faraday Rotation Isolators

Faraday rotation isolators employ magnetically biased ferrite rods to rotate the polarization of an RF signal and are thus susceptible to stray magnetic fields. Stray magnetic fields are defined as those arising from sources external to the isolator and are not magnetic fields generated by the internal magnets used to bias the ferrite. The offending stray fields can arise from other nearby ferrite components but are also common in a variety of mm-wave experiments like dynamic nuclear polarization, particle accelerators, and tokamak fusion reactors [3]. Stray magnetic fields can in principle alter the magnetic bias point in the ferrite and cause under- or over-rotation of the RF signal. The end result is increased insertion loss and decreased isolation. Test results described in this paper will demonstrate that the traditional style isolators sold commercially over the past 50 years are highly susceptible to low-level stray magnetic fields. Even in relatively weak stray fields, the insertion loss can exceed 30 dB rendering the device completely unusable. In contrast, the newer magnetically saturated type isolators, such as those offered by Micro Harmonics, function remarkably well in stray fields up to and exceeding 250 Oe.