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

Takao FUJII  Isao OHTA  Tadashi KAWAI  Yoshihiro KOKUBO  

IEICE TRANSACTIONS on Electronics   Vol.E90-C    No.12    pp.2245-2253
Publication Date: 2007/12/01
Online ISSN: 1745-1353
DOI: 10.1093/ietele/e90-c.12.2245
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Section on Recent Technologies for Microwave and Millimeter-wave Passive Devices)
coplanar waveguides,  directional couplers,  hybrids,  equivalent admittance,  microwave circuits,  slow-wave structure,  

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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 broadband 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.