Array Antenna for Reconfigurations

Abstract

Reconfiguration array antenna is usually achieved in terms of pattern, frequency, and polarization by changing the relative amplitudes, phases of the excitation distribution, and material characteristics present in the array, at the cost of simple and complex feeding networks both. In this chapter, basically different types of array antennas are discussed with its design with reconfigurations. Also, for high generation communication system, a widely used graphene material is taken to get reconfiguration in antenna using array form. There are so many other techniques being used and discussed to get reconfiguration. To get reconfiguration at high frequency, i.e., terahertz frequency, graphene is the best material to use in antenna to solve many applications nowadays compared with only few materials, which is useful design. Here, few designs have been introduced to understand the reconfiguration in array antenna for broadband applications. The proposed chapter represents advancement on the development of multipurpose antennas having different applications in communication, broad banding, detection, etc., in recent research systems.

1. Introduction

A reconfigurable antenna is an antenna having a capability to modify its properties dynamically, in a controlled and reversible manner. The need for multifunctional (e.g., direction finding, beem steering, radar, control, and command), high-performance and cost-effective devices within a confined volume places a greater burden on today’s transmitting and receiving systems. Reconfigurable antennas are a solution to this problem. Reconfiguring an antenna is achieved through deliberately changing its frequency, polarization, or radiation characteristics. Many techniques are there to achieve this change by redistributing antenna currents and thus altering the electromagnetic fields of the antenna’s effective aperture, thereby adapting to changes in environmental conditions or system requirements (i.e., enhanced bandwidth, changes in operating frequency, polarization, and radiation pattern) [1, 2]. This concept can significantly reduce the number of components, hardware complexity, and cost of the communication systems. The first patent on reconfigurable antennas appeared in 1983 by Schaubert [3]. Excellent overview of reconfigurable antennas, with many examples, is given in [4].

The type of reconfiguration and the technique to achieve need to be addressed before designing an antenna. There are four fundamental reconfigurable properties of antenna, i.e., frequency of operation, radiation pattern, polarization, or a combination of any of these properties [4].

These antennas are those antenna systems that are capable of modifying their properties such as frequency, pattern, and polarization dynamically, within a controlled strategy, and with permissible reversibility. The reconfigurability of the antenna array is based on the modification of the geometry of system or behavior of its elements with regular maintenance of the efficiency of the antenna against changes on its environment and mission objectives. Reconfigurable antennas are usually result in low cost, lightweight, low volume but with complexity of maintenance and proper repairing due to being able to offer the same functionalities as multiple conventional antennas [5]. Moreover, the modification of the antenna elements in terms of electrical characteristics causes strong complexity in design of feeding networks results [6, 7, 8], and it is more worse if extra controlling elements are added. Due to this reason, the use of arrays technology by introducing parasitic elements in antenna is very common. Because of these parasitic elements, extra current is induced due to near-field effects without any need of a feeding network in its design, creating beam pattern reconfiguration with a noticeable very low complexity [9, 10]. The most common antennas that use parasitic elements based on linear dipole arrays are known as Yagi-Uda antenna arrays [11]. In such antenna geometry, one more element is added along with parasitic linear dipole known as driven, used in front of those to get reconfiguration in radiation pattern. Also, linear and planar arrays are being used to achieve same goal by use of switch on and off condition, one by one in the same elements [9]. One more antenna that is printed dipole antenna having single parasitic element is used for wide scanning at the cost of little deviation in the gain of antenna, known as beam scanning antenna [12]. As of interest in beam pattern reconfiguration for the antenna, few dielectric materials can also be responsible for reconfiguration by changing its value due to environment behavior and surrounding, giving impressive results for so many applications. As relative dielectric constant of medium, which is gaseous, particularly air is the main ingredient having constant near unity, is basically changed due to change in temperature, pressure, and humidity that can easily be checked during hygrometry measurements using Yagi-Uda antennas [13, 14, 15]. So, the environment effect on dielectric constant is widely used to achieve reconfiguration. Later, this constant can also be changed in the presence of airborne particles and due to pollution in industrial and urban area such as droplets in clouds, which is relevant in today’s antenna design [16]. One more aspect is important during the design of reconfigurable array antenna, which is dynamic range ratio that should be controlled in the amplitude of excitation, which is one main constraint. This dynamic range ratio is the ratio between the maximum and the minimum excitation amplitude of the array elements in antenna, which allows the practical realization of feeding networks, under which noncomplex design is offered with less number of power dividers or the design of micro stripline should become simple. There are so many methods given to work together with pattern and feeding network, which are mainly to reduce the dynamic range ratio, specified value for each excitation amplitude, or controlling phase. Reconfigurability in array antennas is second significant capability that may be requested nowadays from antenna designer to get wide applications. In current scenario, communication systems might in fact have to accomplish multitasking missions, in which the pattern must be reshaped by keeping the constant the value of excitation amplitudes of the elements and also modifying the sole excitation phases. Getting frequency reconfiguration and polarization reconfiguration without changing the parameters of antenna is also very promising and difficult task.

2. Reconfiguration

A reconfigurable antenna is an antenna that is capable of changing frequencies and other characteristics as per requirement. It can be used by using diodes as switches. It is equipped for altering its recurrence and radiation properties progressively, in a controlled and reversible way. So as to give a unique reaction, reconfigurable radio wires incorporate an inward component (for example, RF switches, varactors, mechanical actuators, or tunable materials) that empowers the purposeful redistribution of the RF flows over the receiving wire surface and produces reversible alterations of its properties. Reconfigurable radio wires contrast from savvy reception apparatuses in light of the fact that the reconfiguration component lies inside the receiving wire, and it is opposed to in an outer beam forming system. The reconfiguration ability of reconfigurable receiving wires is utilized to augment the reception apparatus execution in a changing situation or to fulfill changing working prerequisites.

2.1 Types of reconfiguration

Reconfigurable reception apparatuses can be grouped by the radio wire parameter that is powerfully balanced, commonly the recurrence of activity, radiation design, or polarization. Different types of reconfiguration are mentioned as given.

2.1.1 Frequency reconfiguration

Recurrence reconfigurable radio wires can modify their recurrence of activity powerfully. They are especially helpful in circumstances where a few interchanges frameworks merge in light of the fact that the different receiving wires required can be supplanted by a solitary reconfigurable radio wire. Recurrence reconfiguration is by and large accomplished by physical or electrical adjustments to the reception apparatus measurements utilizing RF switches, impedance loading, or tunable materials.

2.1.2 Radiation reconfiguration

Radiation design reconfigurability depends on the purposeful change of the circular circulation of the radiation design. Bar guiding is the most expanded application and comprises of controlling the heading of greatest radiation to boost the receiving wire gain in a connection with cell phones. For example, reconfigurable receiving wires are typically planned utilizing mobile/rotatable structures or switchable and responsively stacked parasitic elements. Over the most recent 10 years, metamaterial-based reconfigurable reception apparatuses have picked up consideration due to their little structure factor, wide shaft guiding extent, and remote applications. Plasma radio wires have additionally been explored as options with tunable directivities.

2.1.3 Polarization reconfiguration

Polarization reconfigurable radio wires are fit for exchanging between various polarizations modes. The ability of exchanging between flat, vertical, and roundabout polarizations can be utilized to decrease polarization bungle misfortunes in versatile gadgets. Polarization reconfigurability can be given by changing the harmony between the various methods of a multimode structure.

2.1.4 Compound reconfiguration

Compound reconfiguration is the capacity of all types of reconfigurations while tuning a few receiving wire parameters, for example, recurrence and radiation design. The most widely recognized use of compound reconfiguration is the mix of recurrence spryness and bar checking to give improved ghostly efficiencies. Compound reconfigurability is accomplished by consolidating in a similar structure distinctive single parameter reconfiguration techniques or by reshaping progressively a pixel surface.

2.2 Techniques for reconfiguration

Four major techniques are available to implement reconfigurable antennas, i.e., electrically reconfigurable antennas, optically reconfigurable antennas, physically reconfigurable antennas, and materially reconfigurable antennas as shown in Figure 1. Electrically reconfigurable antennas use electrical switches such as RF-MEMS, PIN diodes, or varactors to redistribute the surface current and alter the antenna radiating structure topology and/or radiating edges [17, 18, 19, 20]. Optically reconfigurable antennas rely on photoconductive switching elements. Physically reconfigurable antennas can be achieved by altering the structure of the antenna [21, 22]. Finally, reconfigurable antennas can also be implemented through tunable materials such as liquid crystals and graphene as in Figure 1 [4, 23].

Figure 1.

Antenna reconfiguration techniques [4].

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.

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.

Electrical property	RF MEMS	PIN diode	Optical switch
Voltage [V]	20–100	3–5	1.8–1.9
Current [mA]	0.01–0.05	3–20	0–87
Power consumption [mW]	0.05–0.1	5–100	0–50
Switching speed	1–200 μs	1–100 ns	3–9 μs
Isolation (1–10 GHz)	Very high	High	High
Loss (1–10 GHz) [dB]	0.05–0.2	0.3–1.2	0.5–1.5

Table 1.

Electrical properties of electrically and optically switching [4].

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.

Ref	Year/journal	Frequency	Reconfiguration technique	Reconfiguration property	Reconfiguration range
Missaoui et al. [24]	2011 JCEMS	5 GHz	K15 LC	Frequency	4.5–5.5 GHz^#
Papanicolaou et al. [25]	2015 EL	4.77 GHz	K15 LC	Frequency	4.64–4.77 GHz^#
Polycarpou et al. [26]	2014 TAP	5.8 GHZ	E7 LC	Frequency	5.45–5.84 GHz^#
Liu et al. [27]	2008 EL	5 GHz	E7 LC	Frequency	5.3–5.75 GHz^#
Missaoui et al. [28]	2016 DIPED	2.53 GHz	K15 LC	Frequency	2–2.53 GHz^#
Martin et al. [29]	2003 EMC	5 GHz	K15 LC	Frequency	4.94–5.07 GHz^#
Zhao et al. [30]	2017 JPHOT	15.3 GHz	GT3–23001 LC	Frequency and Radiation Pattern	14.5–16.4 GHz, −20° to 20°^*
Pan et al. [31]	2018 ICTCE	6.02 GHz	GT3–23001 LC	Frequency	5.91–6.02 GHz^#
Sethi et al. [32]	2016 APMC	6.6 GHz	DLA 100–100	Frequency	6.4–6.6 GHz^#
Yasir et al. [33]	2017 LAWP	5 GHz	Graphene	Frequency	4.50–5.05 GHz^#
Leng et al. [34]	2015 APSURSI	1.588 GHz	Graphene	Frequency	0.83–1.588 GHz^*

Table 2.

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

^#Simulated and measured result.*Simulated result.

Ref	Year/journal	Frequency	Reconfiguration technique	Reconfiguration property	Reconfiguration range
Yi Huang et al. [35]	2012 TNANO	0.86 THz	Graphene	Radiation pattern	±30°^*
Goyal et al. [36]	2018 MOTL	1.0 THz	Graphene (0.09–0.45 eV)	Frequency	0.96–1.03 THz^*
Dash et al. [37]	2017 iAIM	1.24 THz	Graphene (0.5–0.8 eV)	Frequency	0.98–1.24 THz^*
Anand et al. [38]	2014 ijleo	0.775 THz	Graphene	Frequency	0.725–0.775 THz^*
Wu et al. [39]	2016 EuCAP	1.88 THz	Graphene (0–0.4 eV)	Radiation pattern	2 beam pattern^*
Dash et al. [40]	2018 IET-MAP	2.56 THz	Graphene (0.3–0.6 eV)	Frequency	2.56–4.98 THz^*
Moradi et al. [41]	2018 ijleo	0.51 THz	Graphene (0.3–0.7)	Frequency	0.505–0.52 THz^*
Tamagnone et al. [41]	2012 Appl. Phys. Lett.	1.8 THz	Graphene (0–0.2 eV)	Frequency	0.8–1.8 THz^*
Hlali et al. [42]	2018 PIER C	2.5 and 5 THz	Graphene (0.1–0.4 eV)	Frequency	2.5–2.95 THz, 5.06–5.84 THz^*
Zhang et al. [43]	2018 APCAP	1 THz	Graphene (0.1–2 eV)	Frequency and radiation pattern	1–2 THz^*
Palomino et al. [44]	2015 TAP	1 THz	GT3-23001	Radiation pattern	55°^#

Table 3.

Comparison reconfigurable antennas at THz frequency reported in the literature.

^#Simulated and measured result.*Simulated result.

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 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, 36, 37, 38, 39, 40, 41, 42, 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 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.

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.

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 $\overrightarrow{\mathit{n}}$ indicates the time average directions of the molecules. (c) Liquid phase with lowest molecular order [47].

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.

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_r = 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 f_r, the height of the substrate h, and the permittivity of the dielectric substrate material ε_r (Figure 3).

Figure 3.

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

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.

Figure 4.

Orthographic view of inset feed microstrip patch antenna array with LC substrate.

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 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.

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.

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.

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 k₀. 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}/k₁. 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 substrate with permittivity ε_r = 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 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.

Figure 7.

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

$$\mathrm{L}=\mathrm{m}\frac{{\lambda }_{\mathit{spp}}}{2}=\mathrm{m}\frac{\pi }{\mathit{Re}\left\{k_{\mathit{spp}}\right\}}$$E1

If the length of the graphene patch is integer multiple of half plasmon wavelengths, λ_spp 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.

Figure 8.

Orthographic view of 1 × 4 graphene antenna array.

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, 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 left-hand circular polarization (LHCP) is achieved when only extensions 1 and 2 are ON of all the patch elements.

Figure 9.

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

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.

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.

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.

Figure 12.

Reflection coefficient of (a) single patch antenna and (b) 1 × 4 antenna array.

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.

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.

Parameters	Single patch	1 × 4 patch
Reflection coefficient (dB)	−18.56	−41.11
Gain (dB)	4.02	10.49
Directivity (dB)	7.57	12.69
Efficiency (%)	43	60

Table 4.

Radiation characteristics of single patch and 1 × 4 array 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)

$$\mathit{AR}=E_x/E_y;\text{or}{E}_y/E_x$$E2

where E_x and E_y 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 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 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).

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

Polarization	Axial ratio (dB)	Main beam direction (degree)	Gain (dBi)	Efficiency (%)
LP	30.9	96	10.5	60
LHCP	1.58	91	6.86	38
RHCP	1.58	91	6.77	38

Table 5.

Radiation characteristics of 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 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, 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.

Figure 17.

Effect of variation of graphene chemical potential on resonating frequency.

Patch A	Patch B	Patch C	Patch D	Main beam direction in phi plane (degree)	Gain (dBi)
OFF	OFF	OFF	ON	104	5.61
OFF	OFF	ON	OFF	113	5.03
OFF	OFF	ON	ON	82	6.8
OFF	ON	OFF	OFF	115	4.92
OFF	ON	OFF	ON	89	6.19
OFF	ON	ON	OFF	90	6.85
OFF	ON	ON	ON	89	8.95
ON	OFF	OFF	OFF	75	5.45
ON	OFF	OFF	ON	65	6.9
ON	OFF	ON	OFF	91	6.21
ON	OFF	ON	ON	90	8.86
ON	ON	OFF	OFF	100	6.82
ON	ON	OFF	ON	90	8.78
ON	ON	ON	OFF	91	8.8
ON	ON	ON	ON	90	10.4

Table 6.

Performance parameters of radiation pattern reconfigurable antenna.

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.

Graphene chemical potential (eV)	Frequency (THz)
0.4	0.64
0.5	0.69
0.6	0.70
0.7	0.71
0.8	0.74

Table 7.

Performance parameters of frequency reconfigurable antenna.

Figure 18.

2D polar plot of pattern reconfigurable antenna in orthogonal plane with different arrangements of patch elements.

4. Conclusion

As reconfigurable antenna is very useful nowadays in the similar manner including array technique makes it more applicable in many applications such as satellite communication, biomedical applications, radar systems, smart new generation systems such as 5G and beyond, etc. Graphene, comparatively very new material as per design of antenna in terahertz regime, is most required wonder material for reconfigurable array antenna. Apart from graphene, liquid crystal is also good material to design this antenna in gigahertz range. Also, there are so many other techniques to design and make it useful for various applications. Reconfigurable antennas play important roles in smart and adaptive systems and are the subject of many research studies. They offer several advantages such as multifunctional capabilities, minimized volume requirements, low front-end processing efforts, and good isolation; these makes them well suited to use in wireless applications such as fifth-generation (5G) mobile terminals. There are so many techniques available to reconfigure the antenna properties such as electrical, optical, mechanical, and material-based reconfiguration. Among all the options, material-based reconfigurable antennas are the simplest ones with excellent performance. Where liquid crystal is proved to be suitable candidate in microwave frequency range, the 2D material like graphene is proved to be a real gem in THz frequency range.

Frequency reconfiguration is explained and polarization reconfiguration is discussed with linear, right-hand circular, and left-hand circular polarization. Radiation pattern reconfiguration is also explained with beam variation range of 82°–115°. Efficiency of the proposed structure is slightly less because monolayer graphene has high surface resistance due to very high surface area to volume. But this issue can be eliminated by using multilayer graphene. Hence, graphene-based antennas offer better and simpler reconfiguration technique than any other existing technique at THz frequency.