Finite element solvers for microwave ferrite devices

Microwave ferrite materials are widely used for the design and construction of nonreciprocal phase shift and control components below 40 GHz. In Maxwell's equations a tensor permeability is used to characterise magnetically biased ferrite materials. The entries of this tensor are nonlinear functions of frequency, bias field, material magnetisation and demagnetising effects. This complicated relationship between the dc magnetic bias circuit and the microwave tensor has hindered the development of CAD techniques for this class of component. A new hybrid finite element solver has been developed. This approach utilises both a magnetostatic and a microwave solver and so requires no assumptions about the magnetisation and demagnetising effects. The ferrite is characterised by its hysteresis curve in the magnetostatic solver. The calculated dc magnetic field and magnetisation within the ferrite are then used to evaluate the tensor entries for the microwave solver. This procedure is outlined for a high power differential phase shift circulator which has been developed by TRAK Microwave in Dundee, Scotland.