Bandwidth Enhancement and Optimization of SIW Branch- Line Coupler by Least Squares Method

Directional couplers are passive devices which are used as power dividers and combiners in microwave, millimeter and terahertz systems. By the use of substrate integrated waveguides (SIW) for the design of such devices, the advantages of waveguide and printed circuit (PC) technologies are utilized simultaneously, such as low cost, small size, low transmission and radiation losses. In this paper, the method of least squares is employed for the design and optimization of symmetrical and asymmetrical multi-section branch-line-couplers incorporating impedance matching at its input and output ports. The equivalent circuit of the branch-line coupler is obtained and its transmission and scattering matrices are derived, which are eventually used for the construction of an error function. Its minimization leads to the determination of the dimensions of the coupler and its optimum design. The design specifications of coupler include its power division ratios, port impedances and frequency bandwidth. Three prototype models of branch-line-couplers are designed, namely a 6dB singlesection coupler, a symmetrical 3dB two-section coupler and an asymmetrical 3dB two-section coupler. The latter coupler is fabricated and measured. The designs of couplers are verified by the HFSS full-wave computer software with very good agreement.