In this paper, novel dual-band (DB) branch-line couplers (BLCs) employing a composite right/left handed transmission line (CRLH TL) and fractal geometry are presented for the first time. The CRLH TL, with specified characteristic impedance and phase shift, consists of lumped elements for the left handed (LH) part and fractal-shaped microstrip lines (MLs) for the right handed (RH) part, which can be designed separately. Two designed BLCs are involved in size reduction, one using a 3/2 fractal curve of first iteration, the other constructed based on a hybrid shape of fractal and meandered lines. A miniaturized principle for CRLH TL realization is derived and an exact design method for fractal implementation is developed. For verification, an example coupler was fabricated and measured. Consistent numerical and experimental results confirmed the design concept, showing that the BLCs obtain DB behavior centered at 0.9 GHz and 1.8 GHz respectively with good in-band performance, except for slightly larger coupled insertion loss for the hybrid-shaped BLC case. In addition, the proposed fractal- and hybrid-shaped BLCs obtained a 49.7% and 64.1% size reduction respectively relative to their conventional counterparts working in the lower band. The most important contributions of this article are the demonstration of compatibility between the fractal and CRLH TL techniques and the provision of an alternative approach and a new concept for designing devices.

关键词： Fractal,  Composite right/left handed transmission line (CRLH TL),  Dual-band (DB),  Miniaturization,  Branch-line coupler (BLC)