Capacitive Load Filter

With the development of wireless communications technique, waveguide filters and diplexers with small size, low insertion loss, good frequency-selectivity and farther spurious bands are required in lots of areas. This thesis is to study and design waveguide filters and diplexers with novel structures, which meet these requirements. The main works are as follows:We firstly reviewed filter design theories systematically, and gave some formulae for capacity-loaded transmission lines. Then, a type of microwave cavity structure is proposed to realize maximum capacity-loading and therefore minimize the size of waveguide filters. An optimal method is employed to design a compact filter, which reduces 40% in total length, and has better frequency selection performance, compared with direct coupled resonator filter in the same degree.Meanwhile, we employed this method in evanescent modes and designed series of capacity loaded evanescent filters. We added a metal post in every evanescent mode cavity to attain capacity loading. The length and width of this novel type of filer are only 50% of the conventional filter. Moreover, it has many advantages such as better frequency selectivity and spurious frequency characteristic. This substantiates that the capacitive loading technology is widely useful to miniaturize cavity filters.For deeper understanding, we used capacity-loaded waveguide filters to construct a compact diplexer. Diplexer has three ports, is assembled by two filters to get frequency selection and isolation. A designed diplexer reduces above 50% length compared with the direct coupling cavity diplexer.We then used error correction technique to shun the problem of assembly and debugging. The result shows that this type of method has good effect on improving the adjustability and product yield.