## 3 Beam Steering/Beam Forming Using Reconfigurable Components
### PIN Diodes  
The author of [73] proposed a 2-BIT electronically reconfigurable beam steering transmitarray unit cell. A reconfigurable transmitarray unit cell is composed of six metal layers. The transmitter patch has an O shape slot, and two PIN diodes are mounted to the microstrip patch element. The receiver patch has etched on another O slot patch connected to a transmitter patch with a metallic via that transfer electromagnetic energy to the transmitter. This unit cell achieves 10.1-12.1% 3 dB transmission bandwidth. This paper is further extended in [74; 75] to validate the array parameters of this technique. There are two transmitarrays of 14 \(\times\) 14 and 20 \(\times\) 20, reported as switchable circular and linear polarization. These are 1-bit and 2-bit phase resolution variable (tunable) unit cells. There are two and four PIN diodes mounted that allow transmission phase control for each design, respectively. The 1-bit prototype reconfigurable unit cell's measured gain is 20.8 dBi with a 3 dB transmission bandwidth of 14.6% at 29 GHz. The 2-bit design has a measured gain of 19.8 dBi with a 3 dB bandwidth of 16.2% at 29 GHz. Moreover, a novel reconfigurable Yagi-Vivaldi transmitarray structure was reported in [76; 77] for beam steering of the transmitarray at Ku-band. This reconfigurable transmitarray (RTA) primarily focused on a wide bandwidth and beam scanning at Ku-band. The RTA unit cell contains a tightly coupled microstrip. A Vivaldi antenna based on the transmitter and a Yagi antenna structure etched on it behaves as a transmitter. A pair of anti-parallel diodes are mounted on a slot-line to the microstrip phase shifter to obtain 180\({}^{\circ}\) phase shift with the current reversal mechanism shown in Figure 4. It achieves 14% 1 dB bandwidth of a reconfigurable transmitarray with peak gain 22.3 dBi at 13.6 GHz. The RTA realized 25.6% aperture efficiency with \(\pm\)60 degrees scan angle for both E-plane and H-plane. Two novel linearly polarized reconfigurable design transmitarrays have been investigated with PIN diode at Ku-band in [78; 79]. An asymmetric dipole element PIN diode-based reconfigurable transmitarray is presented in [79]. This novel design has been investigated as a 1-bit dual-band linearly polarized reconfigurable transmitarray at Ku-band, as shown in Figure 5. This element has an active dipole for the receiver and an asymmetric passive dipole for the transmitter 1-bit transmission phase shift, obtained by integrating two diodes on an active patch in the reverse direction with the current reversal mechanism. To reduce the impact of diode resistance and improve insertion loss, two parasitic bypass dipoles alongside the main patches can be added. The dual-polarization parameter is realized by orthogonal interconnecting for the transmitter and receiver. A 10 \(\times\) 10 elements array is designed, manufactured, and tested. It achieves measured aperture efficiency of 22.6% with a peak gain of 18.3 dB at 12.2 GHz. The 2D beam steering feature correspondence realized in a test environment with a scanning angle covers \(\pm\)50 degrees. The maximum measured E-Plane and H-plane losses are 2.9 dB and 3.5 dB, respectively, as shown in Figure 5c. Another novel C-band design of a transmitarray is discussed in [80], and some other applications of this array are explored in [81; 82; 80]. A C-band square ring patch with proximity coupled U-shape resonator transmitarray is proposed. A prototype of a 1-bit beam steering 10 \(\times\) 10 transmitarray with performance is presented. A prototype of a square ring patch with a proximity coupled U-shape resonator with good/bad effects is also demonstrated. The dual-polarized beam scanning effect is experimentally verified in the test setup. The maximum measured gain of the proposed transmitarray is 12 dBi, corresponding to maximum scan loss that reaches 2 dB with a scan angle of \(\pm\)30 degrees.