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Miniature Broad-Band CPW 3-dB Branch-Line Couplers in Slow-Wave Structure

This paper presents some structures of artificial coplanar waveguide with very slow phase velocity and their applications to a design of compact 3-dB branch-line couplers. The slow-wave structure is constructed by periodically loading both of series inductance and shunt capacitance. First, a basic miniature branch-line coupler is designed and consequently considerable size-reduction of about 1/4 is obtained. Next, a broadband design technique is described using open-circuited quarter-wavelength series-stubs added at each port as a matching network. By size-reducing the series-stubs and branchline sections, a very compact broad-band coupler with a good hybrid performance over a wide bandwidth of 31 percent or more is realized. The design concepts and procedures are verified both numerically and experimentally.