Variational principles are efficient CAD tools for planar tunable MICs involving lossy gyrotropic layers

The paper presents an efficient design method for frequency-tunable planar devices using gyrotropic ferrite YIG films. The behaviour of the tunable YI G film is modelled by an equivalent lossy transmission line. Its parameters are the function of the permeability tensor components, which depend on th e frequency and magnitude of the DC biasing magnetic held. We propose two a nalytical variational models for computing these transmission line one- or two-port parameters, depending on their coupling to the planar MIC accesses . Compared with most numerical methods, the two resulting models offer the advantage to be fully analytical and remain valid when losses of the anisot ropic material are taken into account. These two models are able to predict the expected and also the unwanted effects, such as non-reciprocity and lo sses, with a very limited numerical complexity. The efficiency of the propo sed approach is illustrated by comparing theoretical designs using these va riational models with measurements carried out on the various topologies of the tunable one- and two-port planar resonators in the microstrip and slot line technology. Copyright (C) 1999 John Wiley & Sons, Ltd.