Characteristics of microstrip directional coupler on magnetized ferrites

We have studied the physical properties of a high directivity microstrip directional coupler on a single-layered ferrite substrate under the influence of an external dc magnetic field H0. We also present an analysis of the coupled microstrip lines on a ferrite-dielectric substrate. Our method of calculation, which is done in the spectral domain, is based upon a combination of the Hertz vector potentials and the Galerkin numerical technique. The dyadic Green functions for the coupled microstrip lines are also obtained. We consider the even and odd modes of propagation, with the characteristic impedances and normalized phase constants of these artificial structures obtained as a function of their geometry. Special attention is given to the determination of a high directivity for the coupler, by equating the phase velocity of the even and odd modes of propagation which, in turn, is obtained throughout the variation of the applied magnetic field H0.