Overview of Metamaterials-Integrated Antennas for Beam Manipulation Applications: The Two Decades of Progress

Metamaterials (MMs) are synthetic composite structures with superior properties not found in naturally occurring materials. MMs have gained massive attention over the last two decades because of their extraordinary properties, such as negative permittivity and permeability. These materials enable many applications in communication subsystems, especially in the field of antenna design, to enhance gain, bandwidth, and efficiency, reduce the size, and deflect the radiation pattern. The demand for beam-deflection antennas is significant in modern wireless communication research studies due to their importance in enhancing service quality, system security, avoiding interference, and economizing power. The MM structures are usually included in the vicinity of the radiating element or incorporated in the antenna substrate for controlling the radiation pattern. This review study provides an introduction to MMs, focusing on their electromagnetic properties, classification, and design approaches. Furthermore, a detailed study of using the MMs to manipulate the radiation is carried out, where different properties such as the positive/negative refractive index, epsilon-near-zero (ENZ), and mu near-zero (MNZ) are employed to achieve a beam-deflection antenna. Reconfigurable MMs are also loaded to the antenna to achieve multi-directional beam deflection with negligible effect on the antenna’s physical size. Moreover, the gradient-index (GRIN) based on MMs is used to obtain high deflection angles with minor effects on other antenna properties.


I. INTRODUCTION
Metamaterials (MMs) are synthetic composite structures that attracted significant interest over the last two decades due to their peculiar electromagnetic characteristics, such as negative permittivity and permeability [1], [2]. These structures are formed as periodic unit cells whose average size is considerably smaller than the operating wavelength. Their irregular electromagnetic response, the negative permittivity and permeability, was theoretically predicted by Veselago in 1968 [3]. Three decades later, Penury et al. proposed The associate editor coordinating the review of this manuscript and approving it for publication was Giorgio Montisci . the effective medium theory to enable the construction of such materials [4], [5]. The first MM structure, which comprises split-ring resonators (SRR) and metallic wires, was experimentally verified by Smith et al. in 2000 [6]. These unique characteristics of MMs enable many practical applications, including super-lense [7], [8] and cloaking technology [9], [10]. Furthermore, they are widely utilized to realize different electromagnetic devices such as filters, absorbers, and sensors [11]- [13].
With the rapid development of wireless communications and radar systems, antennas are required to be multifunctional, low profile, and cheap to fabricate. These requirements are difficult to be fulfilled using a conventional demonstrating the pattern reconfiguration by focusing on the electrical approach based on switching devices. The radiation pattern manipulation of antennas based on conventional techniques was discussed in [61] and [62], at millimeterwave and terahertz regimes, respectively. Furthermore, the use of artificial materials based on EBG and metasurfaces was discussed in [63], [64]. Unlike the reported ones, this review investigates the utilized methods for implementing beam deflection antennas based on MMs in the light of the available literature. This review paper discusses the use of different properties of MMs such as positive/negative refractive index, epsilon-near-zero (ENZ), and mu near-zero (MNZ) for achieving beam deflection antennas. Moreover, the reconfigurable MMs were also employed for the same purpose, where the beam deflection in multi-direction can be obtained by utilizing the switching devices. Further, the GRIN lens-based MMs were used to manipulate antenna radiation patterns. The proper design of these media can lead to achieving high deflection angles without affecting other antenna features.
This paper is organized as follows. The electromagnetic properties of MMs, along with their classification and design approaches are discussed in Section II. Section III reviews the various techniques to realize reconfigurable MMs and discusses their applications. The concept of GRIN lens based on MMs is introduced in Section IV. A detailed study of the beam deflection antennas based on MMs is presented in Section V, where different properties of MMs, such as positive/negative refractive index, epsilon-near-zero (ENZ), mu near-zero (MNZ), the reconfigurable MMs, and the GRIN lenses are utilized for controlling the radiation pattern. The challenges and future research directions are discussed in Section VI. Section VII concludes the paper.

II. MMS ELECTROMAGNETIC BEHAVIOR AND THEIR CLASSIFICATION
Although MMs are complex structures, their electromagnetic properties are determined through Maxwell's equations, which can assist in demonstrating the features of MMs. Maxwell's equations are given by [65], [66] where E( H ) is the electric (magnetic) field vector, D( B) is the displacement of electric (magnetic) flux density vectors, J ( M ) is the electric (magnetic) current density vector, and ρ e (ρ m ) is the scalar electric (magnetic) charge density. The constitutive relations are used to fully depict the interaction of the electromagnetic fields, which are given by D = ε E and B = µ H . The jω can be used to represent the time derivatives in (1) and (2), where the time dependence is assumed to be of the form e jωt . Here, the Maxwell's equations can be expressed as ∇ × H = jωε E + J (5) where ε(µ) is the permittivity (permeability). Equations (4) and (5) can be presented without sources, electric and magnetic current densities: where k is the wave vector. Additionally, the Poynting power, S, can be used to assess the characteristics of materials, in which the real part is used to demonstrate the energy flow [67]. The S vector has the form of S = 0.5( E × H * ). It is known that both ε and µ are positive in the homogeneous medium, in which the E, H , and k follow the right-hand rule. Therefore, they are called right-handed materials (RHMs). In this case, the S and k have the same direction, as illustrated in top-right quarter of Fig. 1. When ε < 0 and µ < 0, the E, H , and k follow the left-hand rule, and the medium here is the left-handed materials (LHMs) or MMs [68]. In such materials, the S and k are antiparallel, as shown in the bottom-left quarter of Fig 1. LHMs support backward-wave propagation, where the phase velocity and group velocity are antiparallel. The classification of MMs is contingent upon the signs of the two fundamental quantities of material; ε and µ. As claimed by Veselago, some unusual phenomena occur in these materials, such as reversed Snell Law, Cerenkov radiation, and Doppler shift [69]- [71]. Figure 1 shows the material classifications based on ε and µ with the electromagnetic wave interacting with four possible materials. The top-right quarter of Fig. 1 shows the double-positive (DPS) materials with ε > 0 and µ > 0. This type includes the majority of classic dielectric materials. The top-left quarter presents the epsilon negative (ENG) material. This type induces negative ε at some frequencies and positive µ at all frequencies, which is realized by periodic wire structures [72]. The munegative (MNG) material is realized by a periodic structure of split-ring resonators (SRRs), which is presented in the bottom-right quarter [6]. The most interesting one is located at the bottom-left quarter, in which ε and µ are simultaneously negative. This type is known as double-negative materials (DNG)/MMs, which cannot be found naturally.

III. RECONFIGURABLE MMS
Over the last two decades, MMs have been used to construct many novel devices. Their limitations, such as the narrow bandwidth and the losses, can be alleviated by embedding several reconfigurable techniques at the cost of increasing the fabrication complexity and expense. Reconfigurable MM is defined as modifying the electromagnetic response of such structures purposely as a part of the ordinary operation of the device through the change of geometry or material properties. Modern advancements in reconfigurable MMs have increased the possibility of fabricating unique subwavelength (< λ/4) devices with practical functionality [73], [74]. In the literature, various MM structures are reconfigured for diverse purposes at different frequency bands; microwave, millimeter-wave, and terahertz [75]- [82]. Several reconfigurable methods were proposed to create dynamic changes in MMs characteristics, including mechanical, thermal, and electronic. The MMs can be reconfigured mechanically by adjusting the distance between their elements or between the MMs and the substrate, thereby altering the resonance response. However, this method increases the complexity and the fabrication cost [83], [84]. On the other hand, the electromagnetic response of many material characteristics, such as a refractive index, undergoes a change when variable temperatures are applied. Therefore, MMs can be reconfigured thermally by altering their response using an external temperature adjustment [85]. Nevertheless, many tunable devices are operated at the room temperature, and this approach may not be suitable for real-world applications. The electronic approach has gained considerable attention because of its capability to modify MMs' electromagnetic response using different active switches. The development of both electronics and semiconductor industries empowers the combination of active switching components such as a varactor diode, PIN diode, and Micro-electro-mechanical systems (MEMS) with printed circuits. A varactor diode is modeled as a variable capacitance that external bias voltages can control [86], [87]. It represents the most common tuning method because of its simple integration into many MM shapes and the ease of biasing because of using only a single diode to realize the reconfiguration. However, low sensitivity of the capacitance changes limits the tuning range of such a device. On the other hand, the PIN diode is represented by a variable resistance controlled by an external voltage. Both PIN and varactor diodes can be used in MMs for the same purposes [88]- [91]. The MEMS switches offer many benefits over alternative solutions, such as switching capability with high efficiency and isolation, as well as moderate losses [92]- [94].
Nevertheless, high operating voltages, manufacturing cost, and slow switching speed are the main shortcomings limiting their use as RF switches in MMs [73]. It is worth mentioning that there are other methods to reconfigure the MMs, such as including the graphene into the substrate of the unit cell and controlling its response by applying different chemical potentials [95]. Controlling of the electromagnetic response of the MMs enables their use in a wide variety of applications, including: • Reconfigurable filters and antennas; here, the focus is on MMs that are principally designed to tune the resonant response or alter bandwidth 95]- [97]; • Optimized scattering (transmission, reflection, and absorption) of materials at the desired resonant frequency [86], [87], [98].
• Manipulation of wave propagation characteristics such as directivity, gain, and radiation beam [99], [100]. A detailed review of applications of reconfigurable MMs in beam deflection antennas will be presented in Section V.

IV. GRADIENT-INDEX (GRIN) MMS
GRIN lenses received significant attention owing to their ability to focus the radiation. These lenses were proposed as alternatives to the established dielectric counterparts. The homogeneous MMs can be obtained using congruent unit cells arranged in an array form, which are employed in a wide range of applications. On the other hand, if unit cells vary in size, the medium is called inhomogeneous, GRIN, which enables unusual microwave or optical response [101]. The refractive index distribution can be realized based on MMs, which is achieved by the geometric tailoring of the neighboring unit cells. The GRIN based on MMs was proposed theoretically and verified experimentally in [102]- [104], in which the conventional SRR was used as a resonance structure to build a giant inhomogeneous medium. The inherent non-resonant and low-loss behaviors of GRINs enable many practical applications that include but are not limited to the realization of the traditional Luneburg lens [105], [106], and the radar-absorbing system [107], as well as increasing the quantum of energy transfer [108]. Further, this medium can mutate the spherical wave to the planar wave. Thus, the antenna characteristics such as gain, directivity, beamwidth, and sidelobe level can be improved [109]- [111]. The most significant application of the GRINs in the field of antennas is controlling the electromagnetic wave propagation [112]- [114]. The use of GRINs to guide the radiated beam of antennas into pre-defined directions is discussed in Section V.

V. ANTENNA LOADED MMS FOR BEAM DEFLECTION APPLICATION
The radiation pattern is one of the most crucial performance figures of antennas. In some applications, such as wireless base stations, satellite communications, and radar systems, the antennas with beam deflection capability are instrumental as they are able to enhance the performance in terms of system security, avoiding interference, and economizing power. In the literature, MMs were proposed in the last decade for beam tilting applications, which are considered one of the best approaches due to their adjustable refractive index, simplicity of implementation, low profile structure, and potential for integration onto the same substrate, the antenna is implemented on. These artificial materials are placed close to the radiating elements or etched in the same substrate to manipulate the radiation pattern. A detailed review of different beam deflection approaches based on MMs is conducted in this section.

A. BEAM DEFLECTION ANTENNA BASED ON AN ARRAY OF MMs
The MMs are incorporated into various antennas to manipulate the radiation pattern, arranged in the proximity of the radiating part or etched on the substrate. The array of MM is inserted in the path of electromagnetic waves emitting from the radiating element, guiding them toward the desired direction. In this subsection, an array of MMs for performing the beam deflection antennas at different frequency bands is reviewed and discussed, in which several properties were used, such as the positive/negative refractive index, epsilon-near-zero (ENZ), and mu near-zero (MNZ) [27], [57], [58], [115]- [130].
The microwave regime is the most studied part across the spectra for demonstrating the beam deflection antennas using various MM structures. This is due to the availability of the materials and equipment for fabrication and measurement in most laboratories [27], [57], [58], [115]- [124]. The beam deflection mechanism of end-fire antennas depends on the media having different refractive indices, placed in the path of the electromagnetic waves [57], [115]. The simplest way to create a diverse refractive index medium on the limited area of the substrate is by designing suitable MMs. Here, the MM array and the antenna substrate are employed to create different refractive index sections in the path of the electromagnetic waves that undergo a phase change and are then deflected toward the desired direction. The benefits of the MMs are to provide a higher refractive index than that of the substrate and maintain the antenna's size by incorporating them directly into the substrate. Figure 2 presents two media with different refractive indices incorporated in the vicinity of the electromagnetic source, where n and ϕ are the refractive index and deflection angle, respectively. L(d) is the length (width) of the mediums. β 1 and β 2 are the phassbased on the refractive index of the twomediums. The phase difference is given by β= k 0 (n 2 −n 1 )d. The relation between β and the beam deflection angle ϕ is given by β = k 0 Lsin ϕ. The beam deflection angle can be predicted theoretically using the following expression.
In [115], 3 × 4 unit cells of an H-shaped resonator were inserted into the bow-tie antenna substrate for tilting the VOLUME 10, 2022 radiation angle in the E-plane at a C-band. The MM array was shifted from the center of the antenna to the right to create a region with different refractive indices in the path of electromagnetic waves. The configuration of the proposed antenna, including the H-shaped unit cells, is presented in Fig. 3(a). Deflection angle of 17 • was obtained with a gain enhancement of 2.73 dB at 7.5 GHz, as displayed in Fig. 3(b). According to (8), the deflection angle is determined by the difference in the refractive index of the two media (MM array and antenna substrate) and the dimensions of the MM array. By applying the values of L = 21.5 mm, d = 9.7 mm, n 2 = 2.2 and n 1 = 1.6 in (8), the deflection angle ϕ = 17 • was obtained. Figure 3(c) shows no deflection in H-plane as expected as the MM array was incorporated in the E-plane only. The same mechanism was applied by Esmail et al. in [57]. The radiation beam of a dipole antenna was deflected by angles of 25 • and −24 • with gain enhancements of 3 dB and 2.7 dB, respectively, at a sub-6 GHz band of 3.5 GHz. This study demonstrates high deflection angles with good gain improvement. Equation (8) was modified to incorporate more parameters for calculating the deflection angle as follows: The relative permittivity of the substrate is included for a better prediction of deflection angle besides the dimensions and refractive indices of the MMs array and the substrate. By applying the values of L= 59.5 mm, d= 51 mm, n 2 = 2, n 1 = 1.4, and ε r = 2.2 in (9), the positive deflection angle ϕ = 25 • was obtained. The negative deflection angle can be achieved by reversing the values of the refractive indices. Also, at sub-6 GHz, the beam deflection was demonstrated using a 5 × 4 array of double G-shaped resonators (DGRs) placed vertically on the substrate in the elevation plane [116]. The radiation beam was deflected by 35 • in H-plane with the gain improvement of 5 dB at 3.6 GHz.
The transformation optics proposed by Pendry et al. in 2006 [131] give the ability to control electromagnetic wave by transferring it from one space to another. The basic idea is that the Maxwell equations remain form-invariant under coordinate transformations. The result of a particular transformation is a direct link between the permittivity and permeability of the material in different coordinate systems. Several novel transformation optics-based devices have been investigated over the last two decades, such as invisibility cloaks [132], wave rotating [133], and flat reflectors [134]. On the other hand, the authors in [135] demonstrated that two adjacent slabs with the same thickness and opposite signs of the constitutive parameters could act as a zero thickness medium. Here, when a specific volume of space folds to the other one, the corresponding boundaries have identical fields under all circumstances. Therefore, the combination of the transformation optics and folded geometry has led to the construction of many novel devices such as external cloak [136] and EM concentrator [137]. Recently, this approach was used to control electromagnetic waves [58]. Barati et al. [58] merged the conventional SRR and a meander line to form a unit cell for tilting the radiation beam of the patch antenna array. The MMs are arranged in a triangular shape, and placed vertically above the antenna array. The mechanism of beam deflection is based on folded transformation optics. The fabricated patch antenna array and antenna with MMs are displayed in Figs. 4(a) and (b), respectively. The radiation beam was deflected by 15 • at 7.3 GHz, as shown in Figs. 4(c) and (d). However, the gain was reduced by 1.5 dB. This degradation is due to the employing of patch antenna as a source that has a wavefront with curved top surfaces. The patch wavefront can be represented by an infinite number of plane waves with different incident angles. Therefore, for each incident plane wave, the Poynting vector refraction angle will differ in the physical domain, causing power distribution to deviate from its predefined angle and radiate in other directions. Thus, the main lobe amplitude will be decreased to satisfy the conservation law of energy, which leads to a decrease in the gain. The beam tilting antenna based on phase varying of MMs was implemented in [117], [118]. In [117], the MM unit cell consists of capacitive and inductive grids, in which the phase varying MM was created by controlling the gaps between the metallic strips of the capacitive grid. The MM array was mounted above the patch antenna, where the E-plane radiation pattern was deflected in both directions by angles of ±20 • at 10 GHz. On the other hand, the varying phase was implemented by adjusting the inductive grid and fixing the capacitive grid [118]. By mounting these structures above the patch antenna, the radiated beam was deflected by 56 • in the broadside direction at 10 GHz. Jianjia et al. [119] proposed MM based on an electric-LC resonator (ELC) to control the patch antenna's radiation pattern. 5 × 20 ELC unit cells were arranged in ten layers to form a lens, and fed by a patch antenna, as illustrated in Fig. 5(a). Figures 5(b) and (c) show that the radiated beam is deflected by 15 • at 9.3 GHz. However, this arrangement leads to an increase in the overall size of the beam deflection system. The reconfigurable radiation pattern in both positive and negative directions was implemented by Le et al. in [120]. One layer of 5 × 17 folded-line left-handed (FL-LHM) unit cells was located above the patch antenna for tilting the radiated beam by ±20 • with a gain enhancement of 3 dBi at 5.8 GHz. Li et al. [121], introduced NRIM as a phase shifter in the leaky-wave antenna array feeding line for steering the radiated beam by 66 • (58 • −124 • ) in E-plane at 2.4 GHz. Also, in [122], the NRIMs were integrated with the dielectric resonator antenna (DRA) for obtaining positive and negative deflection angles at C-band. By building four layers of the fractal cross ring resonator over the two DRA antennas, the main beam was tilted by angles of ±38 • in E-plane with a gain enhancement of 1 dB over 5−5.5 GHz. The photograph of the proposed antenna with MM layers is shown in Fig. 6(a). The measured radiation patterns of the antenna with and without MM layers in both E-plane and H-plane at 5.2 GHz are depicted in Figs. 6(b) and (c). Here, the positive and negative deflections were obtained by changing the excitation ports of the two DRAs. The beam deflection mechanism was explained based on the Snell's law, sinθ 1 .n air = sinθ 2 .n MMs (10) where the θ 1 (θ 2 ) is the incident angle of the electrometric wave from the air to the MM array (from the MM array to air). The n air (n MMs ) is the refractive index of the air (MM array).
The MMs area denoted by ''b'', as shown in Fig. 7(d). where L is the length of MMs in the x-direction. The beam deflection is due to the phase difference resulting from electromagnetic rays passing through two media of different refractive indices (MMs and antenna substrate). Therefore, simply increasing the length of the MMs along the x-direction results in a larger phase differential and consequently causes a larger deflection angle. Further, the same research group proposed a 2 × 6 arrangement of double split rectangular (DSR) unit cells located vertically on the antenna substrate to achieve beam deflection angles of 17 • and 20 • in H-pane at 3.5 GHz and 5.5 GHz, respectively [124]. The gain enhancements of 4.7 dBi and 5.5 dBi at 3.5 GHz and 5.5 GHz, respectively, were observed when the MM unit cells were arranged symmetrically in the H-plane. The antenna with the DSR unit cells is presented in Fig. 8(a), whereas the obtained deflection angles at both frequencies in H-plane are displayed in Figs. 8(b) and (c).
Recently, the millimeter-wave band has gained growing interest and now appears to be the most likely candidate to deliver the wireless multi-gigabit applications for 5G wireless networks and beyond. This band is expected to present a large bandwidth to meet higher data rate transmissions demand. However, these frequencies suffer from high propagation  loss due to the short wavelength, only allowing short-range communications. High-gain antennas with beam deflection capability constitute an effective and low-cost approach to increase the communication range and provide better beam alignment at this band [138], [139]. MMs are strong candidates to realize millimeter-wave antennas of required characteristics. In the literature, different MM structures were used to deflect the radiation pattern of various millimeterwave antennas [125]- [130]. The radiated beam of the horn antenna was steered by ±15 • at 144 GHz using ε nearzero (ENZ) MM [125]. The ENZs act as a lens realized by hollow rectangular waveguides arranged in an array form at the horn antenna aperture. Low permeability based on SRR was implemented in [126] to deflect the bow-tie antenna's radiation beam by 22 • in H-plane with gain augmentation of 6 dBi at 60 GHz. The beam tilting of a dipole antenna was achieved by Dadgarpour et al. [127] using high refractive-index modified H-shape unit cells. Four slabs of H-shape were installed vertically in the elevation plane of the antenna for performing beam tilting of 30 • in the H-plane over 57-64 GHz. Esmail et al. [128] proposed 3 × 3 square range resonators (SRRs) embedded in the dipole antenna substrate to obtain the deflection in both directions. The E-plane radiation pattern was deflected by angles of 24 • and −25 • with gain improvements of 1.87 dB and 1.84 dB, respectively, at 28 GHz. The beam deflection in both azimuth and elevation planes was implemented using MNZ and ENZ in [129] and [130], respectively. The radiation pattern of the double feed dipole antenna was deflected in E-plane by loading ten slabs of MNZ unit cells vertically on antenna substrate, where two slabs included 6 × 4 unit cells, and eight slabs comprised 10 × 4 unit cells, as presented in Fig. 9(a) [129]. Figure 9(b) shows that the tilting angles of ±35 • were obtained with a gain enhancement of 8 dB at 60 GHz. To obtain E-and H-planes beam deflections, eight slabs of 10 × 7 MNZ unit-cells were positioned in the elevation plane of the four feed dipole antennas, as shown in Fig. 9(c). Figures 9(d) and (e) illustrate that the radiated beam is deflected by 35 • in H-plane and −35 • in E-plane. The beam deflection is due to electromagnetic waves traversing in two media of different refractive indices as predicted by Snell's law. The MM array acts as a lens to deflect the radiation beam in a predefined direction. To implement this approach, the MM array is shifted from the center of the antenna to provide different refractive indices in the path of the electromagnetic waves. In parallel, three slabs, each with six ENZ unit cells, were located above the slot antennas for reconfiguring the radiation in both planes, as shown in Figs. 10(a) and (b) [130]. The slabs were shifted from the center towards (-x)-direction for performing the H-plane deflection. By exciting port 1 and terminating port 2, a tilting angle of −30 • in the H-plane is obtained, as displayed in Fig. 10(c). On the other hand, the deflection angle of −30 • was obtained in the E-plane when the slabs were moved from the center towards (-y)-direction, as shown in Fig. 10(c). Table 1 summarizes the beam deflection antenna using an array of MMs.

B. RECONFIGURABLE MMS FOR BEAM-DEFLECTION ANTENNA
The reconfigurable property is in high demand to increase the functionalities of the devices and reduce their size. As MMs are used in many devices, the reconfigurable property is essential to developing their performance. This property can  be used to deflect the antenna's radiation pattern in multiple directions, in which the size of such a system can be substantially reduced. In this regard, instead of using multiple antennas, the reconfigurable MMs are integrated with the antenna for achieving deflection angles in multi directions. The reconfigurable MMs based PIN diodes for beam deflection applications were proposed in [140]- [145]. The authors in [140], [141], proposed a reconfigurable MMs-based PIN diode for tilting the radiation pattern of dipole antenna at 5G band of 28 GHz. In [140], the E-plane radiation pattern was deflected by 30 • and −27 • depending on the arrangement of reconfigurable contiguous squares resonator (CSRs) on the antenna substrate, as shown in Figs. 11(a) and (b), respectively. In addition, the gain is improved by 1.9 dB and 1.5 dB for the positive and negative deflections, respectively. Three switches were used to reconfigure the MM structure, in which only two states (ON and OFF) were studied to exhibit different refractive indexes in the way of electromagnetic rays. The ideal switches based on copper strips were used as a proof of concept. The theoretical concept of radiation beam deflection relies on two MM configurations of different refractive indices placed in the way of the electromagnetic rays, cf. Figs. 11(c) and (d). In Fig 11(c), 2 × 3 unit cells of reconfigurable MM were inserted in the front of the dipole antenna with overall dimensions of 9.9 mm × 6.4 mm. Two rays of the electromagnetic wave (emitted from source o) travel over the reconfigurable MM and reach two points; m1 (ON state) and m2 (OFF state). The position of all elements in the coordinate plane is revealed in Fig 11(d). The calculation of the array factor (AF) at far-field is used to determine the resultant effect of each ray. The AF can be expressed as AF = 1 + e ikd 1 cosγ 1 + e ikd 2 cosγ 2 (11) where k and d are the wavenumber and the ray length, respectively. The unit vectorsâ m1o andâ m2o that extends from the feed point o to the two MM configurations are given bŷ a m1o = 0.61â x + 0.79â y , and a m2o = −0.33â x + 0.94â y The two electromagnetic rays induce angles of γ 1 and γ 2 with the unit vector of the coordinate plane. The angles γ 1 and γ 2 are obtained by the dot product of unit vectorâ ro withâ m1o andâ m2o , respectively. At θ = 90 • plane and ϕ ∈ (0, 360 • ), the γ 1 and γ 2 are calculated as follows; cos γ 1 = 0.61cosϕ + 0.79sinϕ, and cos γ 2 = −0.33cosϕ + 0.94sinϕ (13) The radiation pattern is determined by multiplying the AF and the dipole antenna element factor (cos((π/2) cos ϕ)/sinϕ) [146], which can be expressed as follows: Radiation pattern = 1 + e ikd 1 cos γ 1 + e ikd 2 cos γ 2 × cos((π/2) cos ϕ)/sinϕ) In parallel, four switches were used to reconfigure the MM structure in [141], in which the beam was deflected by angles of 34 • and −31 • with gain enhancements of 1.7 dB and 1.5 dB, respectively. The authors applied the same beam deflection mechanism as in [140]. Two capacitively-loaded loop (CLL) rings with one switch on each were located on the monopole antenna substrate for tilting the radiation beam by angles of 90 • and 270 • at 1.18 GHz [142]. The first deflection angle was obtained by enabling one switch with ON state and another with OFF state, whereas the second angle was achieved when both switches were OFF. Figures 12(a) and (b) depict the MM antenna's configuration and measured radiation patterns in the E-plane, respectively. In [143], the same research group used two, three, and four SRRs arranged around the monopole antenna for achieving two, three, and four deflection angles, respectively, where one switch was placed in each SRR. Yan et al. [144] proposed three MM unit cells with two diodes to reconfigure the wearable patch antenna at 2.4 GHz. The antenna operated as a conventional patch with a patch-like radiation pattern (broadside radiation pattern) when the diodes were OFF, while it provided a monopole-like radiation pattern if the diodes were ON (omnidirectional radiation pattern). The conventional SRR was  reconfigured based on three PIN diodes and used to tilt the dipole antenna radiation pattern [145]. Two configurations of SRR were obtained based on the state of the three PIN diodes, as shown in Fig. 13(a). The first configuration was obtained when the D1 and D3 were ON and D2 was OFF. The second configuration was achieved by putting the D2 to ON and D1 and D3 to OFF. The ideal switches based on copper strips were used as a proof of concept. As illustrated in Figs. 13(b) and (c), the reconfigurable SRRs (two configurations) were included in the same antenna substrate to obtain a tilting angle of 32 • in the E-plane at 60 GHz. There is no deflection in the H-plane as expected [ Fig. 13(d)]. In addition, this beam deflection is accompanied by a gain enhancement of 2.4 dB. The two configurations create different refractive indices media in the way of electromagnetic rays emitting from the dipole antenna, which are displayed in Fig. 13(e) with lengths l 1 and l 2 . The deflection angle can be predicted by the difference between the phase shift of the wave in two different media. The diagram in Fig. 13(e) shows the beam deflection mechanism. The average ray that enters the two configurations is given by where l a = √ (l 1 /2 + n 2 ) and l b = √ (l 2 /2 + n 2 ). For the proposed structure, l 1 = 2.77, l 2 = 6.23, and n = 2.73, the calculated deflection angle was 30 • , which agreed with the simulated and measured results.
The reconfigurable MMs based on varactor diode were investigated in many studies, and used to control the antenna radiation pattern [147]- [150]. The main advantages of using a varactor diode are the multi-beam obtained by applying a varying bias voltage and using only one diode for unit cell reconfiguration, which helps to reduce the design and fabrication complexity. In [147], Errifi et al. used seven slabs, each with seven omega unit cells arranged vertically above the patch antenna for tilting the radiation pattern at 10 GHz [ Fig. 14(a)]. Two varactor diodes were used to reconfigure each unit cell. By controlling the capacitance values of the two diodes, the positive angle of 30 • and the negative angle of −30 • were obtained at capacitances of 0.1 pF and 3 pF, respectively. This is illustrated in Fig. 14(b). The beam tilting is due to the electromagnetic wave undergoing a phase change when passing through the air and MMs. Xie et al. [148] proposed reconfigurable composite right/lefthanded transmission line (CRLH TL) unit cell based on varactor diode to manipulate the radiation pattern of the leaky-wave antenna (LWA). Varactor diodes were loaded into the unit cells for controlling the phase constant, thereby steering the radiation angle of the antenna from −66 • to 62 • in the broadside direction at 3 GHz. Sun et al. [149] proposed a reconfigurable I-shaped resonator based on varactor diodes to exhibit different refractive indices, which help to guide the antenna radiation beam to the desired direction. The incident wave transmits through the reconfigurable six MM layers and experiences different phase shifts after penetrating each layer. By applying low DC bias voltages to control the diodes of the MM layers, continuous scanning capability in a range of ±30 • at the L-band was attained. The radiation pattern of the horn antenna was controlled by the s-ring structure loaded by varactor diodes [150]. The varactor diodes were used to change the phase of upcoming waves from the horn antenna, thereby steering it in the range of ±40 • at 0.9 GHz. Luo et al. [151] used the MM based on MEMS switches to control the LWA radiation pattern. This report loads LWA by reconfigurable periodic J-shape unit cells to achieve deflection angles of ±15 • at 8 GHz. Later, the same research group proposed tunable MM-based graphene, in which the unit cell consists of a metallic structure embedded by graphene [152]. The tunable MM-based graphene can be classified in two categories: (a) graphene-only plasmonic structures, and (b) graphene/metal hybrid MM structures. In the first type, the graphene is utilized as both the plasmonic media and the reconfigurable media, while in the second type graphene is used only as the reconfigurable media. The second type was used in this report, where the MM unit cell's surface conductivity was tuned by adjusting the chemical potential for producing different refractive indices, which helped to tilt the radiation pattern of the microstrip antenna at the terahertz regime. The beam deflection mechanism was explained based on the Snell's law as in [122]. The MM unit cell and microstrip antenna configurations with two layers of MMs are presented in Figs. 15(a) and (b), respectively. Using one layer of MMs and the chemical potential of 0.5 eV, the deflection angle of −11 • was obtained in the broadside direction (yoz-plane) with a gain enhancement of 0.43 dB. In contrast, the deflection angle was increased to −27 • with a gain improvement of 0.61 dB when using two layers and chemical potential of 0.5 eV at 4.26 THz. Figures 15(c) and (d) display the radiation patterns in the yozplane and xoz-plane, respectively, with different layers and chemical potentials.
The dual-band beam-steering antenna using tunable MM was proposed in [153]. The liquid crystal (LC) extended composite right/left-handed (E-CRLH) MM was arranged in layers to achieve the dual-band beam steering using tunable LC and an external voltage. The photograph of the MM antenna is shown in Fig. 16(a). By applying a bias voltage, the permittivity (PER) varies from 2.53 to 3.11 resulting in beam steering from −16 • to 13 • at 7.2 GHz and −9 • to 17 • at 9.4 GHz. The simulated and measured radiation patterns at 7.2 GHz and 9.4 GHz are presented in Figs. 16(b) and (c). Table 2 summarises the usage of reconfigurable MM for antenna beam deflection.  the SRR was used to tilt the microwave beam. The spherical waves can be converted into planar ones with a deflection angle using the GRINs lens, as illustrated in Fig. 17. The radiation beam can be deflected by θ when the refractive index distribution of the MMs lens follows spatial position function, as follows [102], [154], [155]: where a denotes the distance between the electromagnetic wave source and the GRINs lens, n 0 is the refractive index at the centre location, y = 0 and z = 0. Here, y is the direction of refractive index distribution, and t is the thickness of the GRINs lens. Several studies were carried out to implement beam deflection antennas based on GRINs lens [154]- [159]. The beam deflection antenna based on the GRINs lens was demonstrated theoretically and experimentally by Liu et al. [154]. H-shape and square split-ring resonator were combined to form a unit cell. The MM unit cells were arranged vertically in 17 rows at the aperture of a horn antenna, cf. Fig. 18(a). The unit cells differ in size to exhibit a different refractive index of each row. The electromagnetic wave passes through the MM array and deflects by an angle of 10 • at 15 GHz, as depicted in Fig. 18(b). This is, however, accompanied by the increased complexity and the overall size of the antenna system. In addition, the gain dropped by 0.48 dB. Ma et al. [155] proposed GRINs lens-based interdigital capacitor structure to deflect the radiation beam of the horn antenna by angles of 15 • and 30 • at 8 GHz. The lens antenna and the GRINs lens are shown in Fig. 19(a), whereas the measured electric field distributions that show the two deflection angles are depicted in Figs. 19(b) and (c). Furthermore, the six beam lens antenna was proposed by Aghanejad et al. [156], in which the antenna's main beam was converted into six beams using a non-resonant MM lens.  [157] proposed several 3D GRIN lenses to enhance the radiation characteristics of monopole antenna. These lenses were installed circularly around the monopole, which offered four beams and 10 dB of gain enhancement over 3-8 GHz. Nine layers of different permittivity values were presented by Jia et al. to form a GRIN lens [158]. These values were obtained by drilling holes with varying diameters in each layer. The beam steering was realized by rotating the feed horn along the focal plane. The maximum beam steering of 40 • was obtained in the broadside direction at 12.5 and 14.25 GHz. Although the high steering angle was accomplished, the gain dropped by 2.5 dB.
Dadgarpour et al. [159] discussed the implementation of beam deflection of the dipole antenna using GRINs based on a modified I-shaped resonator. The modified I-shaped resonator was presented in four forms to create variable refractive indices (tape A, B, C, and D). The report studied the beam deflection in three stages. In the first stage, the authors used one antenna with GRINs, in which the radiation pattern was deflected by 26 • in E-plane with a gain improvement of 4 dB, as illustrated in Fig. 20(a). Two antennas were used in the second stage to achieve deflection angles of ±26 • , as depicted in Fig. 20(b). In the last state, high deflection angles of ±56 • were achieved using four antennas loaded by GRINs, as shown in Fig. 20(c). The beam deflection mechanism presented in [140] was applied here. The five rays of an electromagnetic wave travel over the four types of I-shaped in different lengths and directions, as shown in Fig. 20(d). The AF is given by  The angles γ 2 , . . . , γ 5 were obtained by the dot product of the unit vector and the five vectors created by the rays. The far-field radiation pattern was calculated by multiplying the AF and the dipole antenna element factor. Table 3 gives the summary of the beam deflection based on GRINs.
The review carried out in this work leads to several observations concerning the design and realization of beam deflection antennas based on MMs. The following key aspects should be emphasized: • Many techniques were introduced and employed to control the antenna radiation pattern, including electrical and mechanical approaches, and using the phased array antenna-based phase shifters, integrated lens antennas, the traveling wave antennas, and the beam-forming network (BFN) such as the Butler matrix. In phased array antennas, beam steering is made by means of phase shifters and attenuators associated with each one of the radiating elements that conforms to the antenna array.
Designing the phase shifter with advanced electronic controls, achieving a wider scan-angle range, and reducing the feed network complexity and system physical size continue to be significant challenges for antenna designers. On the other hand, the beam-forming based on the Butler matrix comprises phase shifters, couplers, crossovers, and antennas, which are designed separately and then combined for a beam-forming purpose. The design complexity of each stage, large size, and narrow bandwidth are the main drawbacks of such a system. Furthermore, continuous steering is not possible as the radiation beam angle is connected to fixed input ports. In the integrated lens antennas and the traveling wave antennas methods, the large profile structure and unattainable multi-beam scenarios are the main shortcomings. It is expected that existing approaches, including the ones above, will be further improved by using MMs, especially in terms of alleviating the impairments of the traditional techniques. The design of MMs for controlling the antenna radiation pattern is considerably simple compared to the existing approaches, such as the Butler matrix and phased array antennas. The MMs with specific values of constitutive parameters, refractive index, permittivity, or permeability are arranged in the proximity of the radiating element for manipulating its beam. Further, the size of the MM antenna is more minor, whereas solely one antenna is integrated with small size unit cells (< λ/4). Moreover, the bandwidth is not limited, and the continued steering angle can be achieved using the reconfigurable MMs.
Here, the advantages of the MMs over the traditional techniques can be summarized as follows: 1. Simple feed network. A single feed antenna is used for obtaining the beam deflection system. 2. The other antenna features, such as the gain and efficiency, can be enhanced / maintained by proper MMs design. Generally speaking, the antenna performance can be improved by including the MMs with appropriate constitutive parameters. 3. Small physical size system. Unlike the most common beam controlling approaches, Butler matrix and phased array antenna, only one antenna and an array of unit cells (size < λ/4) are integrated to manipulate the radiation pattern. 4. Less design complexity. After designing the antenna, the MM with particular values of constitutive parameters (refractive index, permittivity, or permeability) is only required for realizing the beam controlling system. 5. In contrast to the Butler matrix approach, the bandwidth is not limited, where it relies on the antenna design and even can be enhanced further by including the MMs. 6. Continued angle steering could be achieved by means of the reconfigurable MM based switches, but at the cost of increasing the complexity.
• Over the last two decades, considerable efforts have been devoted to realize beam deflection antennas by exploiting extraordinary characteristics of MMs, such as an array of MMs (with a positive/negative refractive index, ENZ, and MNZ), reconfigurable MMs, and GRIN MMs. In this respect, the degrees of freedom available to the designer when using these unique properties enable a wide range of beam tilting scenarios.
• The efficiency of beam deflection antenna based on MMs depends on the used approach. When including an array of MMs, the deflection angle is contingent upon the number of unit cells and their arrangement in front/above the radiating element. This approach yields a fixed deflection angle due to the stationary function of MMs. By using reconfigurable MMs, the type of switches and the complexity of their bias would affect the deflection angle. Besides, the inherent losses of some of these active devices degrade the system performance. Despite these impairments, this approach can achieve the multi-beam scenarios required by modern communication systems. The distribution of unit cells with a varying refractive index substantially affects the tilting angle when using the GRIN lens. The proper design can achieve a high deflection angle with maintaining/enhancing the gain performance.

VI. THE CHALLENGES AND FUTURE RESEARCH DIRECTIONS
In this section, some challenges in the designing of beam tilting antenna based MMs are presented as follows: • The primary challenge in designing the MM antenna with beam tilting capability arises from the demanding specifications regarding performance expectations, e.g., gain, bandwidth, and efficiency. According to the presented literature, the reports focused on the gain performance, especially at the high-frequency range, where the high gain is instrumental for overcoming the path loss and providing a higher signal-to-noise ratio (SNR) at the receiver. Most works improved the gain performance by including MM array to antennas. However, others showed degradation in the gain for some reason.
On the other hand, the bandwidth and efficiency have not been discussed in most presented reports, and only a few mention such features. Besides the beam tilting, enhancing/maintaining these features (gain, bandwidth, and efficiency) would require more investigation by exploring different MM characteristics and integrating them with various antennas.
• Some characteristics of MMs are in demand for tilting the antenna radiation pattern, such as zero/nearzero refractive index, negative refractive index, ENZ, or MNZ, which are time-consuming to be achieved. Generally speaking, the MM design is based on geometry tailoring where there is no systematic method to attain such features. Here, formulating a solid theory behind MM characteristics that will accelerate the design process with desired performance is necessary.
• As presented in this survey, the reconfigurable MMs are the key to implementing the multi-beam antenna. However, achieving the reconfigurable MMs is challenging, especially at the high-frequency range, even with advanced micro/nanofabrication. This approach can provide a multi-beam antenna at the cost of design complexity. Moreover, the losses associated with some switches could cause degradation in antenna performance.
Some possible future research directions in the beam tilting antenna based MMs are summarized as follows: • The millimeter-wave and terahertz spectrums have recently gained considerable attention due to their large bandwidth and high data rate, which are superior features for delivering the 5G and 6G technologies. The strong directionality and narrow beam of millimeter-wave and terahertz waves may easily cause the problem of local signal coverage holes and leads to signal transmission interruption. Thus, a functional device that realizes intelligent control of the antenna beam is urgently needed in millimeter-wave and terahertz communications, enabling flexible control of the beam's radiation direction and coverage area according to the actual communication requirement. Few reports implemented the beam deflection antenna-based MMs at both spectra. However, most of them controlled the antenna beam in one or two directions. The multi-beam antenna is instrumental in achieving the requirements mentioned above for the current 5G (millimeter-wave) and future 6G (terahertz), which reconfigurable MMs can achieve.
• The reconfigurable MMs-based electrical active switches (PIN and varactor diodes) were implemented in several reports to manipulate electromagnetic waves. Although these switches provide a multi-beam antenna, their high loss could affect other antenna features. The MEMS switches offer many benefits over alternative solutions, such as switching capability with high efficiency and isolation and moderate losses. However, only a few reports used such an approach for beam deflection application based on the presented works. This approach would require more exploration by integrating reconfigurable MM shapes based on MEMS with various antennas to increase the performance further.
• The tunable MM-based graphene is another promising approach, which can avoid the drawbacks of electrical switches. Multiple device structures have been proposed in the literature for realizing tunable MM-based graphene either by graphene-only plasmonic structures or graphene/metal hybrid MM structures. One of the main advantages of graphene resides in the possibility of integration into arbitrary substrates, i.e. low-loss. To the best of our knowledge, a handful of reports used this technique at the high-frequency range. This approach is of interest for realizing a multi-beam antenna without degradation in the other antenna features.
• The losses are crucial and need to be addressed during the design stage to avoid MM antenna performance degradation, especially at the high-frequency range.
Using high conductivity metals such as silver or gold could help reduce the loss; however, ohmic loss of such metals is not avoidable. Recently, dielectric materials have been proposed to design low-loss antennas at a high-frequency range. Also, the dielectric MMs are studied as low-loss media. Therefore, combining both can VOLUME 10, 2022 offer a low-loss beam deflection system. On the other hand, tunable MM-based graphene/MEMS can also provide a low-loss system

VII. CONCLUSION
This review paper provided an introduction to the MMs and discussed their electromagnetic behavior in the light of available literature. Moreover, their classification and design approaches were investigated, and reconfigurable MMs and their applications were introduced. In more depth, this review focused on the capability of MMs to manipulate the radiation characteristics of a broad range of antennas at different frequency spectra. Different MM properties such as the positive/negative refractive index, ENZ, and MNZ were exploited to realize the beam deflection by including their array form in the proximity of the radiating element, or into the same antenna substrate. This approach yielded a fixed deflection angle due to the stationary function of MMs.
On the other hand, the reconfigurable MMs were loaded to antennas for beam deflection applications. Such an approach can be implemented by embedding active switches to the designed gaps of the MMs, which simultaneously deflects the radiation pattern in multiple directions, and reduces the overall size of the beam deflection system. Nevertheless, these switches increase the complexity of the antenna design and exhibit high losses. The GRIN lens is another beam deflection approach, which helps to focus the radiation beam from the antenna into the desired direction. The distribution of the refractive index, which follows the spatial position function, has a substantial effect on the performance of the beam deflection system.