Compact Planar Beamforming Array with Endfire Radiating Elements for 5G Applications

In this paper, a compact $4\times6$ Butler matrix (BM) based on microstrip lines is designed and applied to a linear antenna array. The proposed design creates four beams in four different directions within the 27.5 and 28.5 GHz band. One of the advantages of this BM is a reduction in the size of the beamforming network (BFN). In order to attain this objective, the basic microstrip-based $4\times4$ BM is designed, and then modified to a $4\times6$ BM through a dual-substrate structure to avoid crossing lines using microstrip-to-slotline transitions. The BFN is cascaded with a six-element linear antenna array with endfire radiating elements. The array can be conveniently integrated into the BFN. The resulting design benefits from low-loss characteristics, ease of realization, and low fabrication cost. The array is fabricated and tested, and the experimental results are in good agreement with the simulated ones. The multi-beam antenna size is $5.6 \lambda \times 4.6 \lambda $ including feed lines and feed network, while the new BM design is only $3.5\lambda _{0} \times 1.4\lambda _{0} $ , which is almost half as large as the traditional one. The measured radiation patterns show that the beams cover roughly a spatial range of 90° with a peak active gain of 11 dBi.