Ferrites in waveguides

The gyromagnetic mechanism which controls the permeability of a ferrite at microwave frequencies is investigated theoretically in several simple cases, and the tensor form of the permeability in the general case is indicated. The theory is first applied to circular waveguides containing a ferrite magnetized along the axis. Experimental results are presented which confirm the expected presence of resonance absorption but also show low field losses. A possible explanation of these losses is suggested. Applications of the effects in the construction of isolators and circulators are described. These include resonance and reflection isolators for circularly polarized modes, and also devices which make use of Faraday rotation. The effects of transverse magnetization of the ferrite is then discussed. Applications in rectangular guide in the form of resonance isolators, non-reciprocal phase circulators and field displacement devices are described. Bi-refringent behaviour in circular guide is also described. The relative merits of the different types of isolator and circulator are compared at different frequencies in terms of ease of construction, and performance at one frequency and over a band. The characteristics of most of them can be theoretically calculated from the components of the permeability tensor, and the last part of the paper is devoted to methods for measuring these.