77/79-GHz Forward-Wave Directional Coupler Component Based on Microstrip and SIW for FMCW Radar Application

A low-cost 77/79-GHz ladder-type forward-wave directional coupler (FWDC) is proposed based on microstrip and substrate integrated waveguide (SIW) for frequency modulated continuous wave (FMCW) radar application in this article. The SIW structures are periodically loaded between two parallel microstrip lines. In the circuit topology of the proposed FWDC, the impedance matching between the even/odd modes is achieved. Meanwhile, an approximate constant phase difference (about 12°) between the even and odd modes is designed within 76-81 GHz. The proposed FWDC is modeled by the Bloch-Floquet theorem, and the design procedure is given. Besides, the full-microstrip ladder-type FWDC is discussed and its theoretical analysis demonstrates that such an FWDC with a weak coupling cannot be obtained using the full-microstrip topology. Finally, the proposed FWDC is fabricated by the PCB technology. To measure its performances, W-band fin-line microstrip-waveguide transition and waveguide package are utilized, and three waveguide packaging cases are measured and discussed. Good agreement between the simulations and measurements is achieved.