A compact and broadband backward coupler with high coupling-level based on composite right/left-handed transmission line

Based on composite right/left handed (CRLH) transmission lines (TLs) we propose a backward coupled-line coupler characterized by reduced size and enhanced bandwidth with respect to many other CRLH coupled-line couplers which have been introduced. This symmetric microstrip coupler with quadrature phase difference has also higher coupling-level and wider bandwidth than conventional coupled-line couplers. The proposed CRLH coupled-line coupler has been realized only by conventional TL and grounded stubs without series capacitor (e.g. interdigital capacitor). The design procedure to reach the coupler with predefined coupling-level and characteristic impedance of its ports is clearly presented. The design prototype is a 3-dB microstrip coupler with 0.2 mm spacing between coupled lines over a wide bandwidth of 60% from 2.2 GHz to 4.1 GHz (amplitude balance of 2 dB) centered at 3 GHz. The coupled-line length of the proposed structure is approximately λg/4. To characterize this structure an equivalent circuit model also has been established. Good agreement between the full-wave simulation and equivalent circuit model has been achieved and the effectiveness of the proposed equivalent circuit model is verified.