Figure 3: Simulated E-field plot at 28 GHz for a phased array antenna demonstrating beam-tilt.
Several such phased array antennas are widely shown in the literature. One of the most popular designs is a stacked array antenna. They are capable of generating highly directional tilted beams but are difficult to reconfigure. As an example, a 20 × 20 array of microstrip patch antennas with parasitic elements was presented in [20] to operate at 26 GHz with a half-power beamwidth (HPBW) of 5º and a gain of 30 dBi. Similar techniques are seen frequently in the literature to generate a highly directional high gain beam with reduced sidelobe levels (SLL) at millimeter-wave bands [21]–[25].
Given phased arrays can generate a beam-tilt, several new techniques were explored to achieve a reconfigurable phase shift between the radiating elements. One simple method is providing an individual phase shifter to each radiating element, but it ends up being an extremely bulky and costly system. Such phase shifts can also be generated by a circuit-based feed network to a multi-antenna system to realize a beam-steering (or beam-switching) architecture. Two such popular architectures are known as Butler [26] and Nolen [27] matrices.
A conventional N ×N Butler matrix can control anN- element array and generate N orthogonal beams through different phase distributions. The simplest form of a Butler matrix is designed using a 3 dB quadrature coupler and 45° phase shifters [28], [29]. Such matrices can be optimized for much larger antenna arrays and hence produce wide beam-steering. A complex 8×8 Butler matrix using eight tunable phase shifters was used with a phased array antenna to produce a beam-switching range of 108° with a beam resolution of 2° [26]. Various other works with cascaded Butler matrices are also presented in the literature to facilitate a two-dimensional beam-steering [30]–[34], see Figure 4. Unlike Butler matrices, a Nolen matrix uses couplers with different coefficients along with external phase shifters [35]. They have been demonstrated to be capable of creating multi-beam antenna arrays and 2D-beamforming [27], [36]–[38].