## 3 Beam Steering/Beam Forming Using Reconfigurable Components
### Varactor Diodes  
The literature review also presents the reconfigurable transmitarray (RTA) made up of varactors to accomplish beam forming/beam steering [87; 88; 89; 90; 91; 92]. Varactor diodes are the best solution for continuous tuning by using electronic switches. Varactor diodes are mature technology; they are economical, have low insertion losses, can be modeled easily, and have voltage control switches. There are a wide range of diodes that can be used related to applications shown in Figure 7. It is important to choose the varactor diode that offers the best performance for many applications. A varactor diode capacitance ratio is defined as \(\rm C_{max}/C_{in}\), meaning a large capacitance ratio results in a wide range of tuning capacitance. There is a detail Summary of beam steering reconfigurable transmitarrays with varactor diodes shown in Table 2.  
A varactor diode reconfigurable transmitarray (VD-RTA) is presented in [93; 94] for beam steering performance. Each element of the transmitarray is composed of four substrate Rogger RO4350B double sided layers, shown in Figure 7. One pair can be tuned for 180\({}^{\circ}\) phase shift control, and a whole pair of unit cells duplicated to earn 360\({}^{\circ}\) phase shift. The electronic phase control of the element is obtained by loading each layer with varactor diodes. A theoretical 3 dB bandwidth of the unit cell is 1 GHz at 24.6, as mentioned in this paper, but it reveals that the insertion loss of this paper achieves \(-\)5 dB. Experimental results show that 3 dB insertion losses are quite high up to \(-\)12 dB for this transmitarray. Furthermore, this work is extended in [94] and an array of 6 \(\times\) 6 elements of the abovementioned transmitarray is fabricated. This RTA has the experimental performance of beam scanning capability \(\pm\)50 degrees at 24.6 GHz in both the H-plane and E-plane. Figure 6: (a) Exploded view of the proposed unit cell; (b) simulated results of transmission magnitude and phase shift (reprinted with permission of [84]. Copyright 2020 [PERSON], et al.). Another novel varactor diode and pin diode base phase shifter transmitarray design is reported in [95]. A wideband varactor diode and pin diode base phase shifter reconfigurable transmitter gives continuous phase shift control at 5.8 GHz. This multilayer element structure transmitter (Tx) comprises an H-slot rectangular patch integrated with two diodes. A varactor diode is mounted on the receiver layer to build a 180\({}^{\circ}\) analog phase shifter combined with pin diodes to acquire 360\({}^{\circ}\) electronic phase shift. Theoretical transmission insertion loss includes 0.85 dB to 1.24 dB at 5.8 GHz frequency. A 3 dB bandwidth is obtained up to 17% of the required frequency. A 16 \(\times\) 16 elements prototype of the transmitarray is designed, fabricated, and measured result tested. A maximum peak gain of 23.7 dBi at 5.8 GHz is achieved using the varactor diode-based phase shifter transmitarray. The 2D transmitarray beam scanning performances are maintained for \(\pm\)60 degrees elevation and azimuth planes. In addition, a frequency selective surface (FSS) varactor diode reconfigurable transmitarray with 2D beam scanning ability is reported in [96]. Each layer of the FSS loaded with a varactor diode enables beam scanning of the main beam of the original radiation pattern in both the E-plane and H-plane. A five-layer \(5\times 5\) FSS transmitarray integrated with varactor diodes coupled with a horn antenna feed network has been verified with theoretical and experimental results. Two simulated and fabricated models are developed with a peak gain of 20.2 dBi and 19.9 dBi, respectively, at 5.8 GHz frequency. Experimental results of 2D beam steering are realized approximately \(\pm\)30 degrees in both the azimuth and elevation planes. The proposed RTA shows the potential for use in applications with better economical fabrication cost compared to the most direct alternative beam steering approaches.