**2.3 Comparison between different reconfiguration techniques**

Electronic switching components have been widely used to reconfigure antennas, especially after the appearance of RF-MEMS in 1998. One of the main advantages of such components is their good isolation and low-loss property. While RF-MEMS represents an innovative switching mechanism, its response is slower than PIN diodes and varactors, which have a response on the order of nanoseconds. The ease of integration of such switches into the antenna structure is matched by their nonlinearity effects (capacitive and resistive) and their need for high voltage (RF-MEMS, varactors). The activation of such switches requires biasing lines that may negatively affect the antenna radiation pattern and add more losses. The incorporation of switches increases the complexity of the antenna structure because of the need of additional bypass capacitors and inductors, which increase the power consumption of the whole system.

**Figure 1.** *Antenna reconfiguration techniques [4].*

### *Array Antenna for Reconfigurations DOI: http://dx.doi.org/10.5772/intechopen.106343*

Even though optical switches are not much famous, they offer a reliable reconfiguration mechanism especially in comparison to RF-MEMS. The activation or deactivation of the photoconductive switch by shining light from the laser diode does not produce harmonics and intermodulation distortion due to their linear behavior. Moreover, these switches are integrated into the antenna structure without any complicated biasing lines, which eliminates unwanted interference, losses, and radiation pattern distortion. Despite all these advantages, optical switches exhibit lossy behavior and require a complex activation mechanism. **Table 1** shows a comparison of the characteristics for the different switching techniques used on electrically and optically reconfigurable. The advantages of using physical reconfiguration techniques lie in the fact that they do not require bias lines or resort to laser diodes or optical fibers. However, their disadvantages include slow response, cost, size, power source requirements, and the complex integration of the reconfiguring element into the antenna structure. As for the materially reconfigurable antennas, one major advantage of using liquid crystals is their moderate losses and ease of tuning. Graphene is one other tunable wonder material, which exhibits excellent properties. At submillimeter wave where the footprint of antenna is very small and integrating switches is not possible; we can use these tunable materials to make reconfigurable antennas.

Reconfiguration is possible at GHz as well as THz frequencies as per requirement and applications. Nowadays, advance communication in 5G or more than 5G requires high frequency, which can give high bandwidth for wide range uses. In **Tables 2** and **3**, lists of reconfigurable antennas are given, which are already designed and in use.

The state of the art of research on material-based reconfigurable antennas at GHz frequencies from **Table 2** shows that only few LC materials are explored to achieve reconfiguration. At microwave frequency, graphene is very less used due to high loss. Even LC materials show moderate losses at microwave frequency but because of fabrication limitations, LC is preferred over graphene at microwave frequency. Graphene is much more used than LC at THz frequencies due to unique effect of surface plasmon polariton (SPP) wave. Frequency reconfiguration range depends on the range of permittivity achieved with different LC materials. The designed antenna is based on K15 with no reported gain in literature as given 1 to 0 dBi [24, 25, 29]. A novel design of microstrip patch antenna is proposed based on K15 LC [28]. This antenna has 6.2 dB of gain with good impedance matching. The designed antenna is based on E7 LC reported in [26]. The reported gain is 0.7–1.1 dBi, whereas the gain is not reported [27]. The designed antenna based on GT3-23001 LC is proposed in [30] with the designed structure of 1 4 element antenna array with 12 dBi gain. The antenna applies a double-layer structure with no reported gain [31]. The designed


#### **Table 1.**

*Electrical properties of electrically and optically switching [4].*


**Table 2.** *Comparison of reconfigurable antennas at GHz frequency reported in the literature.*

antenna is based on transparent DLA 100-100 LC with poor impedance matching as reported in [32]. Gain of this structure is not reported. The designed antenna is based on graphene with gain 0.76–2.38 dB in [32] and 0.1–1.9 dB in [33, 34]. At microwave frequency, biased graphene works as a lossy metal, and hence, gain of the antenna reduces due to high power dissipation and increased sheet resistance. So, based on the above literature review, LC is chosen over graphene for the proposed S (2–4 GHz) band antenna for WLAN/WiMAX applications. Comparative analysis of antenna performance and tuning range is done with different LC materials.

The state of the art of research in material-based reconfigurable antennas at THz frequencies from **Table 3** shows that at higher frequency, graphene is preferred over LC. Property of spp waves in graphene help in extreme miniaturization of the antenna size with good performance. Most of the antennas designed in 0–1 THz band are given in [4, 35–43, 45]. The graphene-based microstrip patch antenna is designed on silicon substrate with gain 2.43–4.19 dB, efficiency 48–51%, and return loss 13–26 dB [35]. The graphene-based microstrip patch antenna designed on glass (SiO2) substrate with capacitive-coupled transmission line is given in [36]. Reported gain is 5.08 dB, efficiency is 66%, and return loss is 39.19 dB. The graphene-based patch antenna is designed on silicon substrate with efficiency 16–29%, return loss 23–30 dB, and no reported gain [37]. The graphene-based microstrip patch antenna is designed on polyimide substrate with gain 4.93–5.07 dB, return loss up to 35 dB, and efficiency 86.53–6.85% reported in [4]. The graphene-based Yagi-Uda antenna is designed on glass (SiO2) substrate with gain 6.5 dB and return loss 19 dB [38]. The graphene-based patch antenna designed on glass (SiO2) substrate with directivity 2.99–5.56 dB and return loss 24–34 dB is given in [39]. The graphene-based 1 2 microstrip patch antenna array designed on glass (SiO2) substrate with efficiency 8–43%, return loss 16–30 dB, and directivity 9 dB is given in [40]. The graphene-based patch antenna is


#### **Table 3.**

*Comparison reconfigurable antennas at THz frequency reported in the literature.*

designed on GaAs substrate with efficiency 20% and no reported gain [41]. The graphene-based microstrip patch antenna is designed on Duroid substrate with return loss up to 40 dB and no reported gain [45]. The graphene-based square spiral antenna is designed on quartz substrate with return loss up to 37 dB, impedance bandwidth 26.68%, and no reported gain [42]. The LC-based reflect array (with 54 52 multiresonant cells) in which LC is inserted between quartz and silicon is designed on FR-4 with gain 19.4 dB and return loss 13 dB [43]. So, based on the literature review shown in **Table 3**, it is summarized that graphene-based antennas are much more suitable than LC at THz frequencies. Most of the graphene antennas proposed till now are based on either thick layer of graphene or multilayer graphene with moderate performance. Mostly frequency reconfiguration is reported, very few on radiation pattern, and almost none on polarization reconfiguration. The proposed antenna array is based on monolayer graphene with frequency, polarization, and radiation pattern reconfiguration all together with good performance.
