Non-Folded Capacitor-Based Flexible Phase-Difference Branch-Line Couplers for Butler Matrix Feeding Networks

This paper proposes a flexible phase difference (FPD) coupler without a folded line. The proposed FPD coupler is based on a branch-line structure with four capacitors mounted at each intersection point of the through and coupled arms. To demonstrate the effectiveness of coupler, a $2.4 \mathrm{G H z}$ 45° FPD coupler has been designed and measured. The measurement and simulation results agreed well. The bandwidth of $10-\mathrm{dB}$ reflection coefficient $\left(S_{11}\right)$ is $380 \mathrm{MHz}(15.83 \%$) from the frequency of 2.18 GHz to 2.56 GHz. The transmission coefficient $\left(S_{21}\right)$ is -3.48 dB on average with the imbalance of $\pm 0.42 \mathrm{~dB}$, while the phase difference is -44.84° on average with the imbalance of ±3.18°.