A Novel Dual-Band Branch Line Coupler for Dual-Band Butler Matrix

In this brief, a novel dual-band branch line coupler (BLC) for designing a dual-band Butler matrix (BM) has been proposed. To realize the proposed BLC, a unique dual-band impedance transformer (DBIT) with a derivation of closed-form design equations, capable of performing a dual-band operation for a traditional transmission line (TL) of any electrical length, has been proposed as well. The proposed BLC is different from the existing dual-band BLCs in terms of its phase difference between the output ports, which enables it to play a role equivalent to the combination of a dual-band equal power ±90° BLC and a dual-band ±45° / ±135° phase shifter. The circuit parameters analysis of the proposed BLC versus the frequency ratio has also been studied here. To verify the design method, a dual-band BLC has been designed using Keysight Technologies' advanced design system (ADS), implemented on a Rogers 5870 for 1 GHz and 1.76 GHz operating frequencies, and then tested.