Reflectionless Filter Research Articles

Circularly Polarized Microwave Cavity Filters

A new group of circularly polarized microwave cavity filters is described. With a single circularly polarized cavity, a reflectionless filter is achieved that couples nearly 100 per cent of the energy from the main waveguide at the cavity resonant frequency. Two degenerate cavity modes may be excited, to produce a circularly polarized field, by coupling to the transverse and longitudinal waveguide magnetic fields or to the transverse electric and magnetic waveguide fields. A theoretical analysis is presented as well as experimental results. The loss between the band-pass terminals of the reflectionless circularly polarized filter is identical with the loss in a conventional reflectiontype band-pass filter with the same bandwidth and cavity-wall losses. The null at resonance between the band-elimination terminals of the reflectionless circularly polarized filter is limited only by the asymmetries of the cavity and not by the cavity-wall losses. Design equations and curves are given for eight of the lower order, circularly cylindrical, degenerate cavity modes that are coupled to a rectangular waveguide at the point of circularly polarized waveguide magnetic fields.

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.

Ferrite-Tunable Microwave Cavities and the Introduction of a New Reflectionless, Tunable Microwave Filter

Large ferrite samples are placed in microwave cavities with the object of achieving tunable microwave cavities. Experimental data, at X band frequencies, on ferrite tuning of conventional band-pass and band elimination cavities are presented. Curves of applied dc magnetic field vs cavity resonant frequency, bandwidth, loss, cavity Q, and window coupling coefficients are included. Of particular interest are the results obtained with a new type cavity filter. With a single circularly polarized cavity, a reflectionless filter is achieved which couples nearly 100 per cent of the energy from the main waveguide at the cavity resonant frequency. Experimental results, on the ferrite loading of these circularly polarized cavities, are presented, and a number of practical microwave applications are discussed.