A Two-Stage Beamforming Antenna Using Butler Matrix and Reconfigurable Frequency Selective Surface for Wide Angle Beam Tilting

A two-stage beamforming technique for a wide tilt angle is proposed in this letter. The proposed structure has the potential to overcome the limitations of Butler matrix-based beamforming technique and reconfigurable frequency selective surfaces (FSS)-based beamforming technique, both of which normally lag behind in the present generation of wireless communication due to lesser number of radiating beams. The composite system comprises three parts: a 4 × 4 Butler matrix, an antenna array of 1 × 4 elements, and a reconfigurable FSS layer. Four semi-rigid coaxial cables are employed to connect the Butler matrix to the four-element antenna array. The reconfigurable FSS is mounted on top of antenna layer using air spacers. The FSS superstrate layer consists of 6 × 6 reconfigurable unit cells, which have been further divided into two equal sections to increase the number of radiating beams. Combining the four-port Butler matrix with reconfigurable FSS allows the system to generate 12 unique radiating beams in 12 distinct directions in 3-D space. The proposed system is designed, fabricated, and measured for operating frequency of 5.5 GHz. The proposed system attains a maximum tilt of 72
$^\circ$
and a peak gain of 11.8 dBi.