Field Displacement Type Millimeter-Wave Isolator

When ferrite is exposed to a radio frequency magnetic field superimposed on a steady magnetic field, it exhibits two different values of magnetic permeability depending on the direction of circular polarization of the r. f. magnetic field. If ferrite is placed in a waveguide, a non-reciprocal device - an isolator - can be built. The field-displacement type isolator is relatively simple in construction. The purpose of this thesis was to develop such a simple device which will operate at millimeter wavelengths. In the course of this development, extensive experimental investigations were carried out. Some of the more important results were: The gyromagnetic resonant frequencies increased approximately linearly with increasing applied magnetic fields. The wider the line-width the wider the resonant frequency bandwidth. The frequency band-width could be slightly increased by shaping the ends of the ferrite. The dimensions of ferrite strips were varied in order to determine the optimum length, width and thickness. Polystyrene mounting strips can concentrate the energy and smear the region of circular polarization. An optimum length of the polystryrene [sic] strip was found. Several methods of mounting the ferrite strip in the rectangular waveguide were investigated. The most effective method involved fixing the ferrite strip in parallel with the E plane of the rectangular wave-guide at a certain distance from one side of the wall. Single crystal barium ferrite exhibits a high ratio of backward to forward attenuation but the frequency bandwidth is narrow. Polycrystalline ferrite has a very wide frequency bandwidth. A typical example of a single crystal ferrite isolator constructed exhibited 0.6 db forward loss, 37 db backward loss and 30 mc bandwidth at 58.45 kmc. A typical example of a polycrystalline ferrite isolator constructed had 5 db forward loss 34 db backward loss and 1500 mc band-width at 57.2 kmc. A theoretical investigation of the optimum position and the best figure of merit is included