## 4 Beam Steering/Beam Forming Using Variable Source Antenna  
The beam steering is acquired by changing the position of the primary feed source or using an electronically variable source antenna. To enhance the performance of beam steering, the concept of multi-focal transmitarray is introduced. Generally, two identical feed sources are placed where the phase error becomes minimal. The focal point is 33 degrees for a beam direction provided in bifocal transmitarray. There are some studies reported in the literature that demonstrate beam steering using variable source antennas. The early investigation of variable source antenna beam steering is demonstrated in [110; 111]. Reference [47] presents a beam steering transmission antenna using an electrically variable source antenna. A novel variable source beam scanning transmitarray presents as shown in Figure 11. There is a 3D dielectric printing transmitarray illustrated on paper with a taper section to enhance the bandwidth of the transmitarray. A 50 \(\times\) 50 elements transmitarray with variable source beam scanning capabilities and wide bandwidth transmitarray is validated with measured and experimental results. The measured radiation performance shows 1 dB bandwidth over 23% achieved at 30 GHz. The radiation performance of \(\pm\)27 degrees is obtained in the E-plane at 30 GHz with variable source mechanisms. There is a method to steer the beam by the structurally fixed transmitarray and the switching of the source antenna. Since the wavefront of the source antenna is variable, as described in Figure 3a the beam steering transmission antenna is realized by the combination with the transmission phase of the transmitarray. Five source antennas are selectively operated by applying two SP3T switching elements. Although the beam steering characteristics can be implemented relatively simply, there are also disadvantages of a limited beam steering angle and discretization. To improve such a drawback, a study was conducted to realize a continuous beam steering angle by operating two source antennas and adjusting the position and of each phase [48]. The combination of a two-dimensional metasurface and a planar source antenna capable of one dimensional phase control improves the gain of the existing one dimensional phased array antenna. Moreover, an automotive radar application of a dual-layer 77 GHz printed circuit board transmitarray is presented [49]. There are four SIW antennas used as the primary feed source. Coplanar unit cells are linked with vias and printed on the opposite side of the PCB. The coplanar unit cell was designed with concentric rings to gain a 1-bit phase shift of the transmitarray. Four substrate integrated waveguides were used as source antennas to obtain four beams to obtain a beam scanning range over \(\pm\)15 degrees throughout the  
Figure 11: (**a**) Proposed mechanism of beam scanning transmitarray; (**b**) actual implementation of variable source transmitarray (reprinted with permission of [47]. Copyright 2019 [PERSON], [PERSON], et al.). desired bandwidth. The transmitarray generates 18.5 dB of gain at 76.5 GHz, with good agreement in theoretical and experimental results. Alternatively, in [112], an optically transparent transmitarray is presented with a focal source (horn antenna) replaced with a 2 \(\times\) 2 unit cells aperture couple stack microstrip patch antenna of a seven element array as shown in Figure 12. Beam steering is obtained by switching the position of seven 2 \(\times\) 2 arrays to the next one. The choice of an aperture couple stack microstrip patch array over a horn antenna creates the transparent transmitarray's low profile and light weight. The proposed meshed double ring unit cells achieve a reduced transmission loss and slight dependence on the angle of incidence at the Ka-band. The unit cells contribute to a phase shift of 300\({}^{\circ}\) with 1 dB reduced insertion loss at 28.5 GHz. Theoretical and measured results exhibit beam-wide scanning angles attaining \(\pm\)30 degrees in the azimuth plane at 28.5 GHz.