Study And Design Of Waveguide Directional Filters

A directional filter is a well-matched four-port device,which is popularly used to separate a operation band from another operation band, with little reflection at the input port. It has a selectivity function of a filter and a directional characteristic as a coupler. In a transmit-receive system where the transmitter frequency differs from the receiver frequency, a directional filter is often used as a complementary diplexer. A waveguide directional filter can also be used as a absorptive filter in high-power transmitter, which can filter out the harmonic or other noises, with little reflected power back to high power generator.There are mainly three kinds of directional filters in practical use: half-wavelength or full-wavelength resonator directional filters, traveling-wave-loop directional filters and waveguide directional filters. The first two kinds can be realized by strip-lines or micro-strip-lines which belong to plane structure. They have attractive characteristics such as compact-size and light-weight, however, their applications are usually limited by their high losses and low power-handing capabilities. In the high power or low loss applications, waveguide directional filters are better choices.Conventional waveguide directional filters usually use circularly-polarized modes in one or more than one cylindrical cavities which are coupled to two rectangular waveguides through coupling holes. They can also be constructed by a pair of directly-coupled cavity filters, which are coupled to two rectangular waveguides through coupling holes. The main disadvantage of these two kinds of waveguide directional filters is that their operation bandwidth is usually limited to less than 1%, and their three-dimensional structures are complicated for fabrication and assembling. The research focus of this thesis will be on the design of waveguide directional filters with wide bandwidths and simple structures.In this thesis, we will propose and design two waveguide directional filters. The first is a broadband directional filter, which includes two Riblet couplers and two evanescent mode waveguide filters. It can be used as a broadband complementary duplexer and a absorptive filter. The more important applications of such a broadband directional filter is in the design of multiplexers with large number of channels. This directional filter has a quasi-planar structure, which can be easily fabricated as a base part and a plane cover plate, in which all the circuits are arranged in the base part. This structure can significantly simplify the fabrication process and assembling process. The second directional filter is a waveguide directional filter with simpler structure. The design process and tested results from the prototype will also be presented.