Design of miniaturized branch-line coupler based on novel center-symmetrical spiral-interdigital resonators

A miniaturized branch-line coupler is achieved by using the unit cells of novel center-symmetrical spiral-interdigital resonator (CSSIR) in this article. Due to its intrinsic convoluted structure, CSSIR can be completely embedded in the microstrip line, and a compact transversal dimension is realized. The electromagnetic characteristics of CSSIR are analyzed by Ansoft Designer and equivalent circuit model, and the characteristic of a frequency point with -90-degree phase shift in the passband is proved, and it can be applied to design the quadrature microwave components. According to the design methods of the microstrip branch-line coupler, the 90-degree phase shift 30.5 Ω and 50Ω transmission lines embedded with CSSIR are designed. Instead of the corresponding conventional quarter-wavelength transmission lines, the proposed CSSIR-based transmission lines are utilized to construct the microstrip branch-line coupler. Then, the designed branch-line coupler is fabricated and measured; the measured and simulated results are in good agreement with each other, showing that the operating frequency of this coupler is 3.22GHz, the bandwidth is 3.18GHz-3.32GHz, and this designed branch-line coupler has good electromagnetic performances. Moreover, the size of the designed branch-line coupler is 16.5mm × 11.3mm, and it realizes 54.7% size reduction in comparison with the conventional one. Besides, the proposed branch-line coupler is uniplanar, and no via, defected ground structure or lumped element is introduced.