A Sidelobe-Reduced, Four-Beam Array Antenna Fed by a Modified $4\times4$ Butler Matrix for 5G Applications

A sidelobe-reduced, four-beam array antenna fed by a modified 4 × 4 Butler matrix (BM) is proposed for 5G applications. Two attenuators are utilized in two external channels of the BM to realize a tapered amplitude distribution. In addition, three H-slots are employed as decoupling structures to enhance the isolation between the input ports. The modified BM is then cascaded with a 4 × 7 microstrip comb-line array antenna to establish four steering beams with high gains and reduced sidelobe levels. A prototype of the proposed four-beam array antenna is fabricated and experimentally demonstrated. The overall dimensions of the fabricated prototype are 95 × 32 × 0.254 mm
³
. The total area of the modified BM is only 30 × 32 mm
²
. The measured reflection coefficients and coupling levels between the input ports are less than -15 dB over the target frequency range of 27.525-28.325 GHz. At the center frequency of 27.925 GHz, the measured four beams point toward -16.2°, +40.8°, -39.4°, and +12.6° with corresponding measured peak gains of 17.52, 16.55, 15.74, and 17.87 dBi. The measured peak sidelobe levels for each beam are -16.7, -10.3, -10.9, and -14.5 dB, respectively. Radiation patterns are also presented and discussed.