Gyromagnetic Modes in Waveguide Partially Loaded with Ferrite

It has been shown that, for certain ranges of transverse magnetic field, there are an infinite number of propagating modes in a waveguide completely filled with ferrite, 1 2 3 no matter how small the guide. These modes we will call gyromagnetic modes, since they have no analog in waveguides filled with isotropic material. For symmetrical structures, the modes of these completely filled waveguides show 110 nonreciprocal behavior. It is the intention of the present paper to study a similar set of gyromagnetic modes for a waveguide only partially filled with ferrite. Here there are displayed some interesting nonreciprocal effects, which we will describe. The procedure used will be first to derive explicit expressions for the propagating modes for the partially filled waveguide for a given range 1427 1428 T H E BELL SYSTEM TECHNICAL J O U R N A L , N O V E M B E R 1 9 5 9 of magnetic fields. We will then show how the nonreciprocal modes obtained can be used in a straightforward fashion to construct a novel isolator. Experimental evidence will be presented that indicates that these higher order modes may be excited from the dominant TE mode, even with boundaries that have no variation in the direction of the applied magnetic field. Some theoretical considerations will indicate that finite conductivity in the waveguide walls may be responsible for this coupling. Finally, we will consider the possibility of building purely reactive isolators. These will be shown to be nonexistent from general energy considerations.