Some applications of ferrites to microwave directional couplers, switches, and cavity filters

The paper reviews some of the research and development projects which led to the designs for a ferrite directional coupler, a ferrite microwave switch and ferrite-loaded cavity filters. Directional coupling is accomplished at microwave frequencies by extending a ferrite cylinder through the common wall of two rectangular waveguides joined at the broad face. Experimental data for coupling and directivity are given for two ferrite materials as a function of frequency, biasing field and diameter of the Ferrite post. A microwave circulator utilizing ferrite elements can be converted into a fast-acting microwave switch by placing the ferrite in a pulsed magnetic field. The design of a switch based on the Faraday rotation phenomenon and techniques for the reduction of the short-circuited-turn effect of the waveguide are discussed. Large ferrite samples are placed in microwave cavities with the object of achieving tunable microwave filters. Experimental data at X-band frequencies on variation of cavity resonant frequency, bandwidth, loss, cavity and window coupling are given as a function of applied magnetic field. Of particular interest are the results obtained with a circularly polarized cavity designed to produce a reflectionless filter which couples nearly 100% of the energy from the main waveguide at the cavity resonant frequency.