Q band ferrite isolator for the circular TE01 wave mode

Abstract

The paper discusses possible configurations of ferrite elements for application to an 8 mm isolator operating in the circular TE01 mode. It is concluded that, when using this wave mode at such a frequency, there is one configuration that offers particular advantages—namely that of an eccentrically mounted rod of a highly anisotropic ferrite transversely magnetised to give gyromagnetic resonance at the frequency of operation. The application of perturbation theory to this arrangement shows that there are two alternative positions of the ferrite rod, at different radii, for which the reverse/forward loss ratio is a maximum, and it indicates that the position nearer to the axis of the waveguide should provide the better performance. As the TE01 mode is not dominant in a circular waveguide, the isolator must inevitably operate under conditions capable of providing for the support of a number of other modes at the same time. In the reverse direction of the isolator, the ferrite introduces strong coupling between the TE01 mode and some of these other propagating modes, so that a mode filter forms an essential part of the isolator. Results are given of an experimental investigation showing that a worthwhile degree of isolation can be obtained with a device of this kind.