Design of self-biased hexaferrite waveguide circulators

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Abstract
Self-biased millimeter-wave circulators can be built using hexaferrite materials with high internal anisotropy field, thus avoiding the use of large and heavy permanent magnets. The design principles of these devices are considered thoroughly in the article for both below- and above-resonance circulators. The article treats the classical structure of a half-wave-long ferrite cylinder, two dielectric spacers, and two metal transformers. A simple microwave technique for the measurement of the necessary parameters of the hexaferrite material is used. The choice of an adequate material with a proper value of the internal anisotropy field is discussed. An accurate procedure for the dimensioning of the ferrite insert and the spacers that eliminates the tedious cut-and-try work is described. A way to find the effective internal magnetic field is shown. Simple relations for the calculation of the matching elements are also given. Results on an above-resonance 33 GHz self-biased circulator that support the theory are presented.

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