**3. Design schematic of array antenna for reconfiguration for different materials**

Here, two different materials are being used to get reconfigurability in the array antenna, which is basically liquid crystal and graphene. Both materials are having

properties to achieve high gain compatible with gigahertz as well as terahertz frequency. Liquid crystal gives good response on gigahertz frequency, whereas graphene gives high performance on terahertz frequency.

### **3.1 Using liquid crystal and graphene**

Liquid crystal molecules show an additional state of matter, which lies between a liquid and a solid state [44, 46]. Similarly graphene also features surface plasmon polariton wave at terahertz frequency. Due to the excellent properties and ease of tuning, these materials have become the topic of great interest in the field of reconfigurable antennas. Fundamental properties of liquid crystals and graphene are presented in the first and second sections of this chapter, respectively.

#### *3.1.1 Fundamental of liquid crystal*

The most common states of matter are solid, liquid, and gas depending on the temperature and pressure. In the solid state, the molecules are strongly bonded by intermolecular forces either in a regular order, so-called crystalline solid, or in an irregular order, so-called amorphous solid. The strengths of the intermolecular forces in the LC materials are not homogeneous in all directions because the material is formed by anisotropic molecules. With the increase of temperature, molecules vibrate excessively and breakdown the weak intermolecular bonds, leading to a drop off in the positional order. This middle state is defined as the liquid crystal state, where the molecules exhibit orientational order like that of a solid crystalline but can still flow like a liquid. **Figure 2** shows the schematic representation of a nematic liquid crystal. In an LC bulk, the molecules are aligned parallel to their long axis because of the shape anisotropy. In the macroscopic scale, the time averaged direction of the director is along z axis, which is along the long axis of the molecules. In the both solid and nematic phases, a director can be assigned, whereas the level of molecular ordering is different. This is quantitatively defined by an order parameter S.

#### *3.1.2 Fundamentals of graphene*

The recent discovery of graphene atomically thin layer of graphite by Novoselov, Geim et al. in 2004 brought a period of scientific and technological research. It's been only 15 years to the history of graphene, and it has already attracted considerable attention in fields ranging from material science, nanotechnology to physics, electrical and electronics devices, and circuit applications beyond what is possible with today's silicon chips. Graphene, an extremely good conductor of electricity, comes as an alternative material replacing extensively used materials such as "silicon" or "metals" in field of electronics, in order to further shrink the device size. So, we can clearly say that graphene represents a conceptually new class of materials that are only one atom thick and, on this basis, offers new inroads into low-dimensional physics that has never ceased to surprise and continues to provide a fertile ground for applications. This section reviews the basic theoretical aspects of graphene material and discusses the desirable electromagnetic and mechanical properties possessed by graphene that would assist in providing flexible and reconfigurable antenna structures.

**Figure 2.**

*Schematic of a nematic LC material depending on the temperature: (a) crystalline solid state with the order parameter S = 1. (b) Nematic phase, where n* ! *indicates the time average directions of the molecules. (c) Liquid phase with lowest molecular order [47].*

#### *3.1.3 Reconfigurable array antenna using liquid crystal material*

The antenna design procedure consists of three steps. First, an inset fed patch antenna is designed with copper patch printed on 1.5 mm thick RT/Duroid 5880 substrate with permittivity ε<sup>r</sup> = 2.2, loss tangent = 0.0009. To decide the dimensions of the inset feed patch antenna, we have to first specify the operating resonant frequency fr, the height of the substrate h, and the permittivity of the dielectric substrate material ε<sup>r</sup> (**Figure 3**).

Now LC material is placed between the layers of patch and RT/Duroid 5880 substrate. Once LC material is inserted, the effective permittivity of substrate will have an effect on both RT/Duroid 5880 and LC material. The orthographic view of the antenna is shown in **Figure 4**.

The antenna is designed by means of the simulator ANSYS HFSS. ANSYS HFSS is a 3D full-wave EM solver. This commercial software uses finite element method for solving electromagnetic structures. The feeding structure of the antenna consists of inset fed microstrip transmission line with 50 Ω impedance. To incorporate the array, corporate feed is used.

**Figures 5** and **6** show the return loss S11 plotted as a function of frequency in the range of 2–3 GHz demonstrating the impedance matching conditions for patch antenna array without LC and with LC material. Here, frequency reconfiguration has been achieved. It can be shown that the return loss of the patch antenna array without LC reaches the maximum value of 22.95 dB at resonating frequency 2.45 GHz. After inserting the LC between substrate and patch, the resonating frequency shifts to the small amount and return loss reaches the value of 12.60 dB. Hence, a 1 � 2 antenna array is designed in this work and the comparative investigation of the antenna

**Figure 3.**

*(a) Top view of 1 2 antenna array and (b) front view of 1 2 antenna array.*

#### **Figure 4.**

substrate with LC materials is done. LC material has excellent tuning property with moderate losses at microwave frequency. Based on this excellent property, a GHz reconfigurable patch antenna is designed for WLAN/WiMAX application.

### *3.1.4 Reconfigurable array antenna using graphene material*

In classical antenna theory, the electrical current wave traveling along a PEC antenna propagates at the speed of light in vacuum c0 with wave vector *k*0. On the other hand, the speed of electrical current wave traveling along a graphene antenna is much slower with wave vector *k*spp. This slow propagation of current wave is responsible for the reduction of physical antenna size in accordance with the SPP wave compression factor Re{*k*spp}/*k*1. As a result of these two major differences, we can say that the main factor that differentiates the antennas at microwave frequency from the antennas at THz frequency is plasmonic effect. According to key physics, "Maxwell's equations are scale-invarient" means with scaling we can shift antenna's resonating frequency in microwave range from lower to higher frequency and vice versa with the same performance. However, when we try to apply the same principle to shift the antenna's resonating frequency from microwave frequency to THz frequency, we observe the effect of plasmonic in metals, which give the additional effects. Hence, at higher frequency, Maxwell's equations get modified and we find the "Hybrid electrical-optical solution of Maxwell's equations". Keeping these effects in mind, the antenna design procedure at THz frequency includes few steps. First, a microstrip patch antenna is designed with monolayer graphene patch on 23 μm polyimide

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

**Figure 5.**

*Return loss of 1 2 antenna array (a) without LC, (b) with BL037 LC, (c) GT3-24002 LC, (d) BL006 LC, (e) E7 LC.*

substrate with permittivity ε<sup>r</sup> = 3.5 and loss tangent = 0.008. Monolayer graphene used for antenna design in this chapter exhibits fixed properties of 0.7 eV chemical potential, 1 ps relaxation time, 300 K temperature, and 0.345 graphene layer thickness. To decide the dimensions of graphene THz antenna, guidelines of conventional microstrip patch antenna are followed, but the length of the patch antenna is calculated by following Eq. (1). The feeding structure of the antenna consists of step tapered microstrip transmission line. The designed graphene antenna has attractive features such as extreme miniaturization, flexibility, and high speed. Chemical

**Figure 6.**

*3D radiation pattern for 1 2 antenna array (a) without LC and with LC E7 (b) 3.17 (c) 2.72, GT3-24002 (d) 2.5 (e) 3.3, BL037 (f) 2.35 (g) 2.61, BL006 (h) 2.62.*

potential of graphene can be shifted above or below the Dirac point (thus altering the carrier concentration in the material) by applying a voltage. As graphene provides an extreme freedom to tune its properties, it is very convenient to make such antennas reconfigurable to achieve multifunctionality. The top and frontal views of the antenna are shown in **Figure 7(a)** and **(b)**, respectively.

$$\mathbf{L} = \mathbf{m} \frac{\lambda\_{ppp}}{2} = \mathbf{m} \frac{\pi}{\text{Re}\left\{k\_{ppp}\right\}}\tag{1}$$

If the length of the graphene patch is integer multiple of half plasmon wavelengths, *λspp* as given in Eq. (1), the THz graphene antenna resonates, and the antenna-radiated EM field is maximized. Ultimately, the frequency response and efficiency of the antenna depend on the properties of SPP waves, which in turn depend on the conductivity of graphene [48].

Even though the graphene conductivity is high and tunable, its monoatomic thin layer sheet gives very high surface impedance due to which the gain and efficiency of graphene antennas are very low. To increase the gain and efficiency of the single element antenna, 1 � 4 antenna array is designed. Efficiency of the graphene antenna can also be improved by using multilayer structure, but for the proposed design we will work on monolayer and multielement graphene patch antenna. The orthographic view of 1 � 4 antenna array is shown in **Figure 8**.

With this 1 � 4 antenna array, not only the gain is increased but also the reconfiguration of frequency, radiation pattern, and polarization is achieved, which is demonstrated in **Figure 8**.

#### *3.1.4.1 Design of polarization reconfigurable graphene antenna*

From the properties of graphene given above, the chemical potential of graphene can be shifted above or below the Dirac point (thus altering the carrier concentration in the material) by applying a voltage. This unique tunable property of graphene can be used to design a frequency reconfigurable antenna. When a bias voltage is applied to the designed antenna array as shown in **Figure 9**, the chemical potential of graphene is varied from 0.4 eV to 0.8 eV. With the increase of chemical potential and change of surface conductivity, the dramatical change in antenna properties is observed. Increased chemical potential gives the increase in return loss and shifts of antenna resonance frequency to the higher frequencies. Polarization of the antenna defines the time-varying orientation and relative magnitude of the electric field vector when the antenna is radiating or receiving the EM waves. The polarization can be linear, circular, or elliptical. In terrestrial broadcasting and mobile communications,

**Figure 7.** *(a) Top view and (b) front view of graphene-based microstrip patch antenna.*

#### **Figure 8.**

*Orthographic view of 1 4 graphene antenna array.*

**Figure 9.** *(a) Model of designed LP: (b) RHCP, and (c) LHCP antenna array.*

linear polarization is widely but not exclusively used as the amount of EM radiation is higher and the antennas are assumed to be aligned, while circular polarization is mainly used for satellite-to-earth communication due to the Faraday rotation effect present in the ionosphere. However, when the orientation of the antennas is unknown, circular polarization can be used as it is more robust than linear polarization in such conditions. In this section, we will analyze the use of variable conductivity of graphene as a potential method to switch between different polarizations. In the proposed structure, three scenarios are evaluated based on 1 4 antenna array with a rectangular graphene patch of chemical potential 0.7 eV having all four corners truncated. Graphene extensions are added to substitute the truncated corners as shown in **Figure 9**. Activating or deactivating these extensions allows one to switch between different polarizations. Linear polarizations (LPs) are achieved when all the extensions are ON, means a fixed bias voltage is applied to these corners. Right-hand circular polarization (RHCP) is set when only extensions 3 and 4 are ON, whereas lefthand circular polarization (LHCP) is achieved when only extensions 1 and 2 are ON of all the patch elements.

### *3.1.4.2 Design of frequency reconfigurable graphene antenna*

From the properties of graphene given above, the chemical potential of graphene can be shifted above or below the Dirac point (thus altering the carrier concentration in the material) by applying a voltage. This unique tunable property of graphene can be used to design a frequency reconfigurable antenna. When a bias voltage is applied to the designed antenna array as shown in **Figure 9**, the chemical potential of graphene is varied from 0.4 eV to 0.8 eV. With the increase of chemical potential and change of surface conductivity, the dramatical change in antenna properties is observed. Increased chemical potential gives the increase in return loss and shifts of antenna resonance frequency to the higher frequencies.

### *3.1.4.3 Design of pattern reconfigurable graphene antenna*

Novel properties of graphene easily satisfy our requirements in designing reconfigurable directional antennas for THz communication. Most of the radiation pattern reconfigurable antennas currently exist, work on the leaky-wave theory where in order to tune the radiation patterns, many different bias voltages are required. Apart from these complications, the gain of the antenna is also less. In this project, a new pattern reconfigurable antenna is designed based on graphene patch antenna array. In the given structure, we have achieved a beam reconfiguration with simple 1 � 4 antenna array. Directivity of this structure is pretty good. Due to monolayer graphene patch, the efficiency of the proposed structure is 60%, which can be further increased by using multilayer graphene patch. Four patch elements are used in the proposed antenna array, and an electrostatic bias voltage is applied to these patches to change its resistance modes. When the bias voltage is applied (ON state), the graphene patch is in low resistance mode, and when the bias voltage is not applied (OFF state), the graphene patch is in high resistance mode. This way with two modes and four patch elements, we can have 16 combinations. Naming the patches as A, B, C, and D as shown in **Figure 10** and denoting their OFF and ON state with 0 and 1 respectively for patch A, patch B, patch C, and patch D, we can have different combinations as shown in **Figure 11**.

Starting from the theoretical analysis, applying the structure and calculating the dimensions of the given antenna array, the simulation process is carried out in CST software. Gain of the single element monolayer graphene\-based patch antenna is 4.07 dB with efficiency 43%. To increase the gain and efficiency, we have used 1 � 4 antenna array. **Figure 12** shows the graph of the reflection coefficient, and **Figure 13** shows the simulated radiation pattern of the single element antenna and 1 � 4 antenna array in theta and phi plane. Reflection coefficient graph is plotted as a function of frequency in range of 0.2–1 THz.

With single element patch, the value of reflection coefficient obtained is �18.56 dB at resonating frequency 0.65 THz, whereas with multielement patch, the value of reflection coefficient obtained is �41.11 dB at resonating frequency 0.71 THz.

**Table 4** shows the comparison of radiation characteristics for single element patch and 1 � 4 patch antenna.

The results presented in this section are divided into the results obtained from linear, RHCP, and LHCP reconfigurable antenna with graphene extensions in the corner. Axial ratio (AR) is a factor used to determine the polarization and represents the ratio between the major and the minor magnitudes of the electric field along each of the x- and y-axis [49]. It follows Eq. (2)

$$AR = E\_{\text{x}} / E\_{\text{y}}; \text{or } E\_{\text{y}} / E\_{\text{x}} \tag{2}$$

where Ex and Ey are the electric field components in the x- and y-axis, respectively. Ideally, the AR needed for an antenna to radiate with a circular polarization is AR = 0 dB. However, an AR within 3 dB of this value is still accepted as CP. In contrast, linear

**Figure 10.** *Naming of patch elements in 1* � *4 graphene-based antenna array.*

#### **Figure 11.**

*Different arrangements of states of patch elements in 1 4 antenna array for radiation pattern reconfiguration. Depending upon switching, it is made, when switch is black, it is off condition.*

**Figure 12.** *Reflection coefficient of (a) single patch antenna and (b) 1 4 antenna array.*

polarization is achieved when the AR is as high as possible (electric field is being propagated only on one of the axis). **Figure 5** demonstrate the graph of reflection coefficient and AR for differently polarized antenna. Reflection coefficient graph for

#### **Figure 13.**

*2D polar plot of single patch antenna in (a) phi plane, (b) theta plane and 1 4 antenna array in (c) phi plane, (d) theta plane.*


#### **Table 4.**

*Radiation characteristics of single patch and 1 4 array patch antenna.*

linearly and circularly polarized antenna is plotted as a function of frequency in the range of 0.2–1 THz with almost same response. Slight shift in resonating frequency is observed when polarization is changed from linear to circular. Axial ratio graph is plotted as a function of theta angle to show the polarization effect in the main beam direction. Reference line shown in the axial ratio graph represents the main beam direction of the antenna at resonating frequency. Axial ratio obtained for linearly polarized antenna and circularly polarized antenna is 30 dB and 1.58 dB, respectively.

To visualize the sense of CP radiation of the proposed antenna, simulated surface current on the patch elements for four phase angles 0° (time t = 0), 90° (t = T/4), and 270° (t = T/2) is shown in **Figure 5**. Surface current distribution of the LHCP geometry is shown in **Figure 14(a)** with the orientation of surface current from 0° to 270° at 0.515 THz. For 0° phase reference, the currents are directed to the z direction. For 90° phase difference, the current flow is dominated by x direction. For 180° phase angle, the current flow is oppositely directed to 0°, which is in +z direction. Again for 270° the current flow is +x directed. Similarly, the current distribution can be explained for RHCP from **Figure 14**(**b**). Surface current distribution direction for the phase 0° is equal in magnitude and opposite in direction to 180°. Same is the case of surface current distribution direction at 90° and 270°. Hence, the criteria for CP are satisfied. As the direction of view is chosen as +y-axis, the direction of rotation of current is anticlockwise and the sense of polarization is confirmed as LHCP. The direction of rotation of current is clockwise and the sense of polarization is confirmed as RHCP (**Figures 15** and **16**).

**Table 5** shows the summary of radiation characteristics of proposed polarization reconfigurable antenna.

To analyze the property of frequency reconfigurable antenna, the proposed structure is simulated in CST software, and the result is shown in **Figure 17**. Chemical potential of all the four patch elements of 1 4 antenna array are changed simultaneously from 0.4 eV to 0.8 eV. Obtained simulated result is summarized in

**Figure 14.** *Reflection coefficient of 1 4 antenna array for (a) LP, (b) LHCP, and (c) RHCP.*

**Figure 15.** *Axial Ratio of 1 4 antenna array for (a) LP, (b) LHCP, and (c) RHCP.*

**Figure 16.**

*Simulated surface current distributions on single patch element of the antenna array at 0.515 THz for (a) LHCP, (b) RHCP at different phases (i) 0°, (ii) 90°, (iii) 180°, and (iv) 270°.*

**Table 6**. Shift in frequency is observed from 0.64 THz to 0.74 THz with change of chemical potential from 0.4 eV to 0.8 eV. It is clear that the achieved frequency band, which is covered by the proposed frequency reconfigurable antenna, is 0.64–0.74 THz,


**Table 5.**

*Radiation characteristics of polarization reconfigurable antenna.*

**Figure 17.** *Effect of variation of graphene chemical potential on resonating frequency.*

which gives the bandwidth of 100 GHz, which is high enough. Hence, with such reconfigurable antennas, we can easily remove the drawback of narrow band antennas.

**Table 7** depicts the simulated radiation characteristics of pattern reconfigurable antenna. The main beam direction of the antenna array can be reconfigured based on the ON/OFF state of each patch element of 1 4 antenna array as shown in **Figure 10**. With two modes (ON/OFF) and four patch elements, we can have 16 combinations, but here we can ignore the case when all patch elements are in OFF state because in this case the surface resistance of graphene will be very high for all the patches, and hence, the antenna will not radiate. **Figure 18** shows the 2D polar plot in orthogonal plane for rest of the 15 cases.
