**Metasurfaces for Reconfiguration of Multi-Polarization Antennas and Van Atta Reflector Arrays**

School of Electrical, Computer, and Energy Engineering, Arizona State University, Tempe, AZ 85287, USA; msalharb@asu.edu (M.A.); malyahy3@asu.edu (M.A.A.); subramanian.ramalingam@asu.edu (S.R.); aymodi1@asu.edu (A.Y.M.); CRAIG.BIRTCHER@asu.edu (C.R.B.)


Received: 9 July 2020; Accepted: 4 August 2020; Published: 6 August 2020

**Abstract:** This paper discusses the application of metasurfaces for three different classes of antennas: reconfiguration of surface-wave antenna arrays, realization of high-gain polarization-reconfigurable leaky-wave antennas (LWAs), and performance enhancement of van Atta retrodirective reflectors. The proposed surface-wave antenna is designed by embedding four square ring elements within a metasurface, which improves matching and enhances the gain when compared to conventional square-ring arrays. The design for linear polarization comprises of a 1 × 4 arrangement of ring elements, with a 0.56*λ* spacing, placed amidst periodic patches. A 2 × 2 arrangement of ring elements is utilized for reconfiguration from linear to circular polarization, where a similar peak gain with better port isolation is realized. A prototype of the 2 × 2 array is fabricated and measured; a good agreement is observed between simulations and measurements. In addition, the concepts of the design of polarization-diverse holographic metasurface LWAs that form a pencil beam in the desired direction with a reconfigurable polarization are discussed. Moreover, recent developments incorporating polarization-reconfigurability in metasurface LWAs are briefly reviewed. In the end, the theory of van Atta arrays is outlined and their monostatic RCS is reviewed. A conventional retrodirective array is designed using aperture-coupled patch antennas with a microstrip-line feeding network, where the scattering from the structure itself degrades the performance of the reflector. This is followed by the integration of judiciously synthesized metasurfaces to reconfigure and improve the performance of retrodirective reflectarrays by removing the above-mentioned undesired scattering from the structure.

**Keywords:** reconfigurable antennas; surface wave array antenna; metasurface ground plane; low-profile; square-ring antennas; van Atta reflector; RCS reduction; structural mode scattering; metasurfaces; polarization reconfigurability; leaky waves

#### **1. Introduction**

Metasurfaces are two-dimensional, less bulky, and low-loss equivalents of metamaterials [1]. They have been popular as they are suitable for a variety of applications that include wavefront shaping and control [2,3], beamforming using high-gain leaky-wave antennas (LWAs) [4–6], radar cross section (RCS) reduction [7–9], and design of low-profile antennas [10,11]. These metasurfaces are realized as an array of patches on a grounded dielectric substrate, and they are easy to fabricate. By changing the geometry of these patches, their surface impedance and reflection phase can be controlled. A zero reflection phase can be realized at a particular frequency, thus mimicking a perfect magnetic conductor (PMC). Hence, they have been utilized to enhance the performance of antennas,

in addition to the design of reconfigurable antennas. This paper details the utilization of metasurfaces to design and enhance the performance of three types of antennas: surface-wave antenna arrays, polarization-reconfigurable holographic LWAs, and van Atta retrodirective reflectors.

#### *1.1. Surface-Wave Antenna Arrays*

Proper excitation of surface waves enhances the overall antenna performance [12,13]. Consequently, several printed antennas have been reported, where the influence of surface waves has been examined. Monopole-like radiation has been achieved using a circular patch in the vicinity of uniform (square) and non-uniform metasurfaces [14,15]. Similarly, surface waves have been excited utilizing a square ring element surrounded by anisotropic metasurfaces, thus yielding broadside radiation [11]. Another design achieving broadside radiation has been realized by placing a diamond-shaped patch in the vicinity of square-patch metasurfaces [16].

To further improve the radiation performance, the analysis has been expanded to include multiple radiators in the vicinities of metasurfaces to form array metasurface antennas [16,17]. The reported designs require a specific spacing between the metasurface supercell to maintain and realize a high broadside gain. This spacing degrades the aperture efficiency and reduces the maximum attained gain. As it will be demonstrated in this paper, the spacings between the metasurface supercell can be reduced without disturbing the functionality of the design. The aforementioned designs are linearly polarized and have a wide range of potential low-profile applications. Circular polarization can be realized by placing a linear-to-circular metasurface superstrate above radiating elements [18]. This approach requires a certain spacing between the radiating element and the metasurface superstrate, which disturbs the overall height-profile. The second approach is to excite surface waves in two orthogonal planes that share the same magnitude and 90◦ phase difference [19–21]. A truncated square patch element positioned in close proximity to square-patch metasurfaces resulted in 7 dB peak gain, a fractional bandwidth of 45.6%, and 3-dB axial ratio (AR) bandwidth of 23.4% [19]. Another design implemented a planar slot to excite rectangular-patches metasurfaces and led to an average gain of 5.8 dB, an impedance bandwidth of 33.7%, and a 3-dB AR bandwidth of 16.5% [20]. The low peak gain observed in the above-mentioned designs is improved to 12 dB by forming a 2 × 2 array of truncated patch elements positioned below square-patches metasurface [21]. This achieved gain can be further promoted by exciting surface waves along with the active element's fundamental mode, which will be demonstrated.

In the first part of this paper, which corresponds to surface-wave antenna arrays, an array of multiple square ring elements embedded within a metasurface is presented. With different array configurations, linear and circular polarizations are achieved.

#### *1.2. Polarization-Reconfigurable Holographic LWAs*

Metasurfaces have facilitated the realization of surface-impedance modulated periodic LWAs that are capable of forming fan or pencil beams with high gains and narrow beamwidths. One-dimensional LWAs form a fan beam in the desired angular direction, whereas two-dimensional LWAs form a pencil beam. The angular direction of the formed beam is determined by the modulation parameters of average surface reactance and period of modulation. One-dimensional periodic metasurface LWAs were realized in [22], while two-dimensional LWAs were designed using the principle of holography in [4].

The polarization of the formed beam is dependent on the polarization of the leaky-wave mode; TM-mode leaky waves result in vertically-polarized beams, while TE-mode leads to horizontally polarized [23]. Circularly-polarized beams were realized by employing a spiral surface impedance modulation in [24]. The design of scalar holographic metasurfaces to form a pencil beam in the desired direction with a desired polarization was detailed in [25]. Tensor impedance surfaces with amplitude, phase, and polarization control were proposed in [26].

The suitability of these metasurfaces to communication-based applications necessitates the formation of beams with reconfigurable polarizations. The design of polarization-diverse metasurfaces that can form a pencil beam in the desired direction with a horizontal, vertical, or circular polarization is discussed in [6]. The polarization state was reconfigured by changing the source of excitation. A similar approach was used to realize dual reconfigurable polarizations using tensor impedance surfaces, and recently proposed in [27]. Another recently-proposed design achieves polarization reconfigurability by using polarization-insensitive holographic surfaces, where the reconfiguration is facilitated by diodes on the surface-wave launchers [28].

The second part of this paper reviews the procedures to design a multi-polarization metasurface LWA. The existing techniques are reviewed and potential new techniques are proposed.

#### *1.3. Van Atta Retrodirective Reflectors*

In many radar and communication systems, reflectors are utilized because of their ability to maximize the re-radiation towards the direction of wave incidence. Such a response can be achieved using a retrodirective reflector, first proposed in [29]. The retrodirective reflector is an antenna array whose elements are interconnected by transmission lines such that the received signal is then reradiated towards the direction of incidence [30–32]. In automotive collision avoidance systems, a high scattered field can be achieved only toward near-normal directions to the target surface. Thus, by equipping vehicles and road obstacles with retrodirective reflectors, the self-phasing feature of such reflectors will increase scattering beamwidths and targets will become more visible [32]. Furthermore, retrodirective reflectors have been investigated for the application of wireless power transfer [33–35]. A device that requires wireless power can send a beacon signal that is then received, amplified, and sent back to the user by the retrodirective antenna array.

Generally, retrodirective arrays can be synthesized with basic radiating elements such as dipoles [31]. In addition, they can be designed using patch antennas [36–38]. However, due to the high-backscattering from the structure (consisting of antenna arrays), it degrades the performance of the retrodirective reflector by destructively interfering with the desired reradiated fields. Thus, a low-backscattering array of long slots was investigated [39], and it was shown that this reflector could reradiate fields without interference from the array structure's scattering. However, a retrodirective reflector comprised of a patch antenna array is a better option due to the simplicity of design and the low profile of patch antenna elements if the high backscattering by the patch antenna array can be mitigated. Thus, artificial magnetic conductor (AMC) technology is utilized to reconfigure the performance of such antenna arrays by reducing scattering from such structures [40].

In the third part of this paper, a van Atta retrodirective reflector with a smoother backscattering pattern is synthesized and developed using a two-dimensional microstrip-antenna array. Conventional retrodirective reflectors are sensitive to the interference by the fields scattering from the antenna structure. Using a virtual feeding network, structural mode scattering is identified and canceled using metasurfaces.

#### **2. Surface-Wave Antenna Arrays**

This section details the first of the three areas, where metasurfaces are used to realize and enhance the antenna performance. In this section, a 1 × 4 arrangement of square ring elements is printed on the same plane with periodic patches to achieve linear polarization. With a spacing of 0.56*λ<sup>o</sup>* between the elements, a high realized gain with high aperture efficiency is attained. The polarization can be reconfigured from linear to circular using a 2 × 2 structure with a spacing of 0.85*λ<sup>o</sup>* between the ring elements. After analyzing the port isolations between the ports, a simple parallel feeding network is designed and incorporated with the structures. Due to the similar characteristics of the two arrays, only the 2 × 2 circular polarized structure is experimentally validated.
