Broadband Frequency-Independent Beamforming Networks for Multibeam Antenna Arrays

This letter presents a broadband frequency-independent beamforming network (BFN) for multibeam antenna arrays. The BFN consists of power dividers with a frequency-dependent power-dividing ratio (PDR), crossovers, and a conventional wideband Butler matrix. The analyses and designs of 2 × 4 and 4 × 8 BFNs are given. For proof-of-concept demonstration, a 2 × 4 BFN with two frequency-independent beams operating at 2-3.75 GHz is designed and fabricated. Within the bandwidth, the measured return loss and isolation are better than 14.8 dB and 19.6 dB, respectively. The time-delay differences between output ports are -36∼-82 and 40∼83 ps, when two input ports are excited, respectively. Based on the measured S-parameters of 2 × 4 BFN, the calculated beam directions are -21.8° and 22.4° at 2.87 GHz, while beam direction variation and array factor variation from 2 to 3.75 GHz (60.9%) are 2.9° and 1.7 dB, respectively.