A Novel Compact Broadband Quasi-Twisted Branch Line Coupler Based on a Double-Layered Microstrip Line

A novel quasi-twisted miniaturized wideband branch line coupler (BLC) is proposed. The design is based on bisecting the conventional microstrip line BLC transversely and folding bisected sections on double-layered substrates with a common ground plane in between. The input and output terminals, each with a length of λ_g/4, and the pair of quarter-wavelength horizontal parallel arms are converted into a Z-shaped meandered microstrip line in the designed structure. Conversely, the pair of quarter-wavelength vertical arms are halved into two lines and transformed into a periodically loaded slow-wave structure. The bisected parts of the BLC are placed on the opposite side of the doubled-layer substrate and connected through four vias passing through the common ground plane. This technique enabled a compact BLC size of 6.4 × 18 mm², which corresponds to a surface area miniaturization by ~50% as compared to the classical BLC size of 10 × 23 mm² at 6 GHz. Moreover, the attained relative bandwidth is 73.9% (4.6-10 GHz) for S11, S33, S21, and the phase difference between outputs (∠S21 - ∠S41). However, if a coupling parameter (S41) of up to -7.5 dB is considered, then the relative bandwidth reduces to 53.9% (4.6-10 GHz) for port 1 as the input. Similarly, for port 3 as the input, the obtained bandwidth is 75.8% (4.5-10 GHz) for S33, S11, S43, and the phase difference between outputs (∠S43 - ∠S23). Likewise, this bandwidth reduces to 56% (4.5-8 GHz) when a coupling parameter (S23) of up to -7.5 dB is considered. In contrast, the relative bandwidth for the ordinary BLC is 41% at the same resonant frequency. The circuit is constructed on a double-layered low-cost FR4 substrate with a relative permittivity of 4.3 and a loss tangent of 0.025. An isolation of -13 dB was realized in both S₁₃ and S₃₁ demonstrating an excellent performance. The transmission coefficients between input/output ports S₂₁, S₄₁, S₂₃, and S₄₃ are between -3.1 dB to -3.5 dB at a frequency of 6 GHz. Finally, the proposed BLC provides phase differences between output ports of 90.5° and 94.8° at a frequency of 6 GHz when the input ports 1 and 3 are excited, respectively. The presented design offers the potential of being utilized as a unit cell for building a Butler matrix (BM) for sub-6 GHz 5G beamforming networks.