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].