Stress-related insertion loss in longitudinal-field ferrite devices

Some microwave ferrimagnetic materials, e. g. compositions of the yttrium-iron garnet family, are known to exhibit sensitivity to intrinsic or externally applied mechanical stress. This stress can cause the magnetic properties of the material to be inhomogeneous, resulting in undesirable insertion loss increases in devices such as dual-mode ferrite phase shifters that use variable longitudinal-field bias. Typically the insertion loss increases appear as "spikes" at low bias field magnitudes. This paper presents analyses based on a transmission-line model for the ideal and stress-distorted cases. A conclusion is that the existence of stress-induced inhomogeneties can break the degeneracy of normal modes in the zero-bias condition, causing the observed behavior. A method is suggested for screening ferrite rod samples to determine suitability for use in dual-mode phase shifters.