In all ferromagnetic and ferrimagnetic materials, there is interaction between microwave fields and the spinning electron, but it is only in ferrites and garnets, which are electrically insulating, that useful interaction between the magnetic properties of the material and electromagnetic waves can be obtained. A classical description of the spinning electron will be used to give a pictorial explanation of the way magnetic materials act on microwave fields.

In this chapter, we consider the propagating conditions for a circularly symmetric system of ferrite in circular waveguide which is magnetised in some direction perpendicular to the direction of propagation along the waveguide. Four different systems of transverse magnetisation are shown. The radially magnetised ferrite tube is almost impossible to realise in practice and will not be considered further. The circumferentially magnetised ferrite tube is used in the design of a latching phase changer. Using the square-loop magnetic property of microwave ferrite materials, the ferrite may be magnetised to saturation circumferentially by passing a pulse of electric current through a wire threading the tube. The ferrite will then remain magnetised to remanence until it is demagnetised. The analysis of the circumferentially magnetised ferrite tube is given. Both the simple transverse magnetisation and the four-pole field have been used to provide birefringence to the microwave field in the waveguide. The effect of the transverse magnetisation is described, the effect of the four-pole magnetisation is described and the governing equations for a theoretical analysis are given.

The effect of a transversely magnetised ferrite medium on a plane wave is discussed. However, for the dominant TE₁₀-mode in rectangular waveguide, the field approximates to a plane wave only at the centre of the waveguide. To get a maximum interaction between the ferrite material and a microwave field the magnetic component of the microwave field needs to be circularly polarised in a plane perpendicular to the direction of magnetisation in the ferrite.

This chapter discusses: scattering matrix theory, junction cavity, E-plane junction, isocirculator, matching networks, and striplines. These are all considered in relation to the Y-junction circulator.

In the millimetre wave frequency range, scaling becomes a problem so that copies of microwave devices become difficult to manufacture with adequate accuracy. Then special techniques suitable only for millimetre wave devices become necessary. This chapter concentrates on those techniques which are peculiar to ferrite devices operating at millimetre wave frequencies. The rest of this section discusses the different transmission structures that have been developed particularly for use as millimetre wave lines. In the next section, we consider the limitations due to the fact that the ferrite material is much less efficient as a gyromagnetic material at these higher frequencies. For free-space propagation, there are a number of atmospheric gas absorption frequencies which limit the range of useful propagation, but there are also windows at 35, 94, 140 and 230 GHz, which is why there are a large number of publications concentrating particularly on these frequencies.

The microwave properties of ferrite and other dielectric materials are often measured by observing the change in resonant frequency and Q-factor of a microwave resonant cavity when a small sample of ferrite or other material is inserted. If the ferrite sample is small, it will have only a small effect on the undisturbed fields in the cavity. Calculations can be simplified, if it is assumed that the fields external to the ferrite body are unchanged from those of the empty cavity and it is only the fields in the ferrite body which are different. The theory is called perturbation theory because it is assumed that the ferrite makes only a small perturbation to the fields existing in the unperturbed activity. Perturbation theory, as it is applied to a resonant cavity, is given in this chapter. Then it is applied to the measurement of ferrite material properties. The particular cavity systems suitable for the measurement of permittivity and tensor permeability are described and theoretical calculations are given.