Capacitor-based Branch-line Couplers for Simple Configuration of Flexible Phase-difference Butler Matrices

Flexible phase difference (FPD) couplers are microwave devices attractive for multiple beam feeding networks to increase the scanning capability of multibeam antenna systems. However, conventional couplers suffer from design issues due to the existence of folded lines in their structure. To overcome these issues, this paper proposes an FPD coupler without a folded line in its arm. The proposed coupler is configured by branch-line couplers with four capacitors at each intersection point of the through and coupled arms. To demonstrate the effectiveness of proposed configuration, a 2.4 GHz 45° FPD coupler is designed and implemented to construct an FPD 4 × 4 Butler matrix with phase differences of ±45° and ±135°. The measurement results show good performance of matrix. When input port 1 is excited, the reflection coefficient is lower than −10 dB within the 280 MHz bandwidth (11.86%) from the frequency of 2.19 GHz to 2.47 GHz. Over the bandwidth, the amplitude imbalance of transmission coefficient and the phase imbalance of phase difference are ±2.3 dB and ±20.1°, respectively. Moreover, the FPD 4 × 4 Butler matrix based on the proposed coupler exhibits a simple configuration.