High Power Reciprocal Ferrite Switches Using Latching Faraday Rotators

Introduction

Faraday Rotation is the simplest and most intuitive way in which microwaves interact with a magnetized ferrite medium. Consequently it is also one of the oldest phenomena used for construction of practical microwave ferrite components. The approach was already well known by 1956 when Ohm presented a paper discussing an optimized version of a four-port circulator based on Faraday Rotation.

This device used a slender rod of ferrite "floating" in a cylinder of polystyrene foam, which in turn was fitted into a circular waveguide with a sleeve of solid polystyrene lining the interior walls of the guide. The operating frequency range was 10.7 to 11.7 GHz., and the length of the ferrite rod was 3.85 inches (97.79 mm.), plus linear tapers of one inch (25.4 mm.) at each end. A bias field of 200 Oersteds was required to magnetize the ferrite to the level required for optimum performance. An insertion loss of 0.19 dB. and isolation greater than 30 dB. were achieved.

Clearly, the sequence of coupled ports can be inverted by reversing the direction of the applied longitudinal magnetic bias field, and so this type of circulator can be adapted to form a nonreciprocal switch. The amount of rotation required is only ±45°, so that with proper choice of material and careful design, a low insertion loss is possible. In addition to being nonreciprocal rather than reciprocal, this basic configuration could be improved by (a) better heat transfer from the ferrite rod to the external structure, and (b) a closed magnetic path for the bias field to permit fast, latching operation. An intrinsic problem with this kind of device is the fact that deviations of the Faraday Rotation angle from the optimum 45° value cause power to be coupled to the isolated port. The magnitude of the coupled power increases as the sine of the error angle. It is evident that in order to maintain an isolation of at least 30 dB., the error angle must not be allowed to exceed about 1.8 degrees, which corresponds to a deviation tolerance of about 4 percent of the optimum rotation amount. This is a really difficult level of precision to achieve and maintain over a significant frequency band and over a wide operating temperature range.