Experimental Characterization and Empirical Beam Steering Analysis of a 28 GHz 1-Bit 8x8 RIS Reflectarray via Conductivity Pattern Modulation

Experimental Characterization and Empirical Beam Steering Analysis of a 28 GHz 1-Bit 8x8 RIS Reflectarray via Conductivity Pattern Modulation Vasiliki Papapostolou School of Electrical and Computer Engineering Aristotle University of Thessaloniki Thessaloniki, Greece [email protected] Yiorgos Makris Dept. of Electrical and Computer Engineering Jonsson School of Engineering and Computer Science The University of Texas at Dallas Richardson, USA [email protected] Alkis Hatzopoulos School of Electrical and Computer Engineering Aristotle University of Thessaloniki Thessaloniki, Greece [email protected] Zaharias D. Zaharis School of Computing and Engineering University of Huddersfield Huddersfield, United Kingdom School of Electrical and Computer Engineering Aristotle University of Thessaloniki Thessaloniki, Greece [email protected] Abstract— This work investigates the beam steering capabilities of a small-to-medium-sized Reconfigurable Intelligent Surface (RIS) consisting of an 8×8 array of simplified 1-bit reflective unit cells operating at 28 GHz. Each unit cell is composed of two rectangular patches connected by a single PIN diode, forming a compact and fabrication-friendly design that omits vias and complex bias circuitry. The array is illuminated under normal incidence by a vertically oriented horn antenna, and various binary ON/OFF control patterns are applied to modulate the reflectivity states of the unit cells. The resulting far-field radiation patterns are measured experimentally to assess the extent to which the direction of the main lobe can be steered solely by modifying the conductivity pattern across the surface. Particular emphasis is placed on identifying empirical rules that relate the spatial geometry of the applied patterns to the observed beam direction. The results demonstrate that even modest-sized RIS arrays can achieve effective beam steering through appropriately structured binary control, offering insight into low-complexity RIS design strategies. Keywords— Reconfigurable Intelligent Surface (RIS), Reflectarray, 28 GHz, 1-bit unit cell, beam steering, pattern control I. INTRODUCTION Reconfigurable Intelligent Surfaces (RIS) have recently emerged as a transformative technology for next-generation wireless communications, such as 6G and beyond. By enabling programmable control over the electromagnetic (EM) environment, RIS can dynamically manipulate incident wavefronts to enhance signal quality, reduce interference, and improve coverage, all with minimal energy consumption and hardware complexity. Unlike traditional active beamforming systems that rely on power-hungry RF chains, RIS-based systems utilize passive or semi-passive elements to reflect or redirect incoming waves toward desired directions, offering a promising solution for energy-efficient and cost-effective beam control [1], [2]. A key factor in the performance of a RIS lies in the design of its unit cells, which are typically structured to impart controllable phase shifts upon reflection. Among various design strategies, 1-bit unit cells—capable of switching between two discrete reflection phase states (typically with a 180° difference)—have gained significant interest due to their simplicity, reduced control requirements, and ease of fabrication [3]. These unit cells often utilize PIN diodes as binary switches to alter the conductive state of the surface. Prior works have explored multilayer and via-integrated designs to implement such configurations, achieving wideband performance and compact form factors [4]. However, these implementations often increase manufacturing complexity and cost. In this work, we propose and experimentally evaluate a simplified 1-bit RIS architecture composed of an 8×8 array of reflective unit cells operating at 28 GHz. Each unit cell follows the geometry presented in [4], consisting of two rectangular metallic patches connected by a single PIN diode, but eliminates the use of vias and multilayer substrates in favor of a single-layer dielectric and ground plane. The RIS is illuminated under normal incidence by a vertically oriented horn antenna, and various binary ON/OFF control patterns are applied across the array to examine the resulting far-field radiation patterns. The full-wave electromagnetic simulations of the unit cell and array configurations were performed in Ansys HFSS to verify reflection behavior and evaluate the expected beam steering performance. The main focus of this study is to assess the ability of such a modest-sized RIS to steer the main lobe direction through simple binary conductivity modulation and to derive empirical relationships linking the spatial geometry of the applied patterns to the observed beam steering angles. The remainder of this paper is organized as follows: Section II describes the design of the simplified unit cell and its implementation, the RIS array configuration and experimental setup. Section III presents and analyzes the measured radiation patterns under various control patterns. Finally, Section IV concludes the paper and discusses future research directions. II. RIS ARCHITECTURE A. Unit Cell Design RISs have emerged as a promising technology for dynamic control of electromagnetic wave propagation in wireless communication systems. Central to their functionality is the design of the individual unit cells, which must balance electromagnetic performance with fabrication complexity and cost. In this work, a simplified 1-bit reflective unit cell (Fig. 1) is employed following the geometry and design principles presented in [4]. Each unit cell consists of two rectangular metallic patches connected via a PIN diode (MACOM MADP-000907-14020), enabling binary control of the reflected phase. The diode acts as a switch to alter the surface current distribution, thereby producing a reflective phase difference of approximately 90° between the ON and OFF states at 28 GHz. Unlike the original multilayer configuration proposed in [4], the unit cell in this study is fabricated on a single dielectric substrate (Rogers RO4350B) with thickness 1.116 mm, equivalent to the total thickness used in the reference design. A continuous ground plane is placed beneath the substrate. The geometrical dimensions of the unit cell are maintained as in [4], with a unit cell footprint of 2.5 mm × 4 mm (0.23λ × 0.37λ at 28 GHz). The top layer contains the two metallic patches, while the PIN diode is centrally placed to connect them. In simulations, the ON state of the diode is modeled as a series LR circuit (Ron = 4.2 Ω, Lon = 0.05 nH), while the OFF state is modeled as a parallel RC network (Roff = 1.2 kΩ, Coff = 33 fF) in series with an inductance Loff = 62 nH, consistent with the equivalent circuit presented in [4, Fig. 8] and extracted from the diode's S parameters that can be found in the official website of the MACOM. This simplified structure is particularly well-suited for experimental prototyping and low-complexity RIS reflective arrays. Fig. 1. Geometry of the proposed 1-bit RIS unit cell. B. RIS Array Configuration and Excitation The RIS implemented in this work consists of an 8×8 array of identical 1-bit reflective unit cells, resulting in a total of 64 controllable elements. The unit cells are arranged with a center-to-center spacing of λ/2, where λ = 10.71 mm corresponds to the operating frequency of 28 GHz. This subwavelength spacing is chosen to minimize grating lobes and ensure constructive interference in the far field. The RIS is illuminated by a standard pyramidal (rectangular) horn antenna operating at 28 GHz with a nominal gain of 12 dBi. The horn is positioned directly above the geometric center of the RIS at a vertical distance of 60 mm (Fig. 2) and its longer aperture dimension is oriented parallel to the x-axis, as shown in the top view of Fig. 3. Fig. 2. Side view with horn & RIS. Fig. 3. Top view with horn & RIS. III. BEAM STEERING PERFORMANCE A. Beam Steering Behavior under Binary Control Patterns To investigate the beam steering capabilities of the proposed RIS, multiple binary ON/OFF control patterns were applied to the 8×8 array of unit cells. As described earlier, the RIS consists of 64 controllable reflective elements with a center-to-center spacing of λ/2 (λ = 10.71 mm at 28 GHz), illuminated by a standard rectangular horn antenna operating at 28 GHz. The horn is positioned vertically and aligned with the geometric center of the RIS, at a distance of 60 mm from its surface, satisfying the Fraunhofer far-field criterion and ensuring that the array is illuminated by an approximately uniform plane wave. All simulations were carried out in ANSYS HFSS under this fixed configuration. Each pattern corresponds to a distinct conductivity distribution across the unit cells, which in turn alters the overall reflection phase profile of the surface and steers the main lobe of the reflected radiation. For each applied conductivity pattern, the far-field radiation was extracted and represented in terms of threedimensional polar plots of the total gain, allowing the main lobe direction and side-lobe distribution to be systematically observed. With this setup, the influence of different spatial ON/OFF patterns on the steering of the reflected beam can be clearly assessed and compared. All radiation patterns presented in this section correspond to far-field 3D polar plots of the total gain, with the corresponding ON/OFF control distributions shown as insets. We begin by examining the two extreme cases in which all unit cells are switched either to the ON state (Fig. 4), or to the OFF state (Fig. 5). As expected, both configurations yield a broadside reflection (θ = 0°), with no observable beam steering. Here, θ (theta) and φ (phi) denote the polar and azimuth angles, respectively, in the spherical coordinate system adopted for the far-field analysis. These two reference cases serve as benchmarks for subsequent comparisons. Fig. 4. All unit cells ON, broadside reflection (θ = 0°). Fig. 5. All unit cells OFF, broadside reflection (θ = 0°). Next, we apply linear ON/OFF distributions. In the first case, all unit cells in the bottom row were set to ON while the rest remained OFF (Fig. 6), whereas in the second case, only the leftmost column was ON (Fig. 7). In both configurations, the main lobe is deflected by approximately θ = 5° toward the side of the OFF elements—φ = 180° for the bottom-row excitation and φ = 90° for the left-column excitation. These results indicate that linear discontinuities in the surface conductivity can introduce small but measurable steering effects aligned with the axis of asymmetry. Fig. 6. Bottom row ON and others OFF, main lobe deflected by approximately θ = 5° toward φ = 180°. Fig. 7. Leftmost column ON and others OFF, main lobe deflected by approximately θ = 5° toward φ = 90°. A diagonal (corner-based) ON/OFF configuration (Fig. 8), was then tested, producing a stronger deflection of the main beam toward the corner direction, with steering observed at φ = 130° and θ = 30°. Additionally, a circular-inspired pattern, adapted from [5], yielded a more moderate deflection of θ = 7° at φ = 90° (Fig. 9). The latter suggests that radially shaped distributions can bias the reflection toward the direction of the pattern’s displacement. Building upon the circular configuration and following the design concept in [6], several modified patterns were investigated. One arrangement produced a steering angle of θ = 30° at φ = 90° (Fig. 10). By rotating the same pattern by 90° around the z-axis, the beam was redirected to φ = 0° at θ = 27° (Fig. 11). A further variation of this design yielded an increased steering angle of θ = 33° at φ = 90° (Fig. 12). When this distribution was slightly shifted downward along the xaxis, the main lobe steered toward φ = 65° with θ = 33° (Fig. 13), again following the direction of the geometric displacement. Fig. 8. Diagonal corner-based configuration, producing beam steering at θ = 30°, φ = 130°. Fig. 9. Circular-inspired pattern producing θ = 7° at φ = 90°. In the same design family, a variant achieved θ = 35° at φ = 90° (Fig. 14). Among all tested configurations, the largest single-lobe steering was obtained with the pattern in Fig. 15, producing θ = 36° at φ = 90°. Finally, a related variant with reduced spacing between OFF elements (Fig. 16), resulted in two dominant lobes, located at φ = 65° and φ = 110° with θ ≈ 42°, highlighting a trade-off between larger steering angles and single-lobe integrity. Fig. 10. Modified circular pattern (based on Fig. 9) producing θ = 30° at φ = 90°. Fig. 11. Rotated version of Fig. 10, redirecting the beam to θ = 27° at φ = 0°. Fig. 12. Further variation of the Fig. 10 design, producing θ = 33° at φ = 90°. Fig. 13. Downward-shifted variant of Fig. 12, steering the main lobe to θ = 33° at φ = 65°. Fig. 14. Optimized circular-inspired pattern producing the largest single-lobe steering at θ = 35°, φ = 90°. Fig. 15. Configuration achieving maximum single-lobe steering at θ = 36°, φ = 90°. Overall, the results demonstrate that the spatial geometry of the ON/OFF distribution critically determines both the steering direction and the quality of the main lobe. Linear boundaries introduce small angular deviations, while more complex and asymmetric patterns can produce significantly larger steering angles—up to θ = 36° in this study—though often at the expense of increased side-lobe levels or even beam splitting. These observations provide the basis for empirical design rules that link the geometry of binary control patterns to the resulting beam steering behavior. Fig. 16. Variant of Fig. 15 with reduced OFF-element spacing, producing two dominant lobes at θ = 42°, φ = 65° and φ = 110°. B. Toward an Empirical Beam Steering Rule Beyond the analysis of individual ON/OFF distributions, the simulation results allow us to infer general trends that can be used to formulate empirical rules for RIS beam steering. Specifically, the steering direction and angle of the main lobe are strongly correlated with the spatial geometry of the applied conductivity pattern. First, linear asymmetries—such as activating a single row or column of unit cells—introduce small deflections (≈5°) toward the side opposite the ON region. This suggests that abrupt boundaries in the conductivity distribution bias the reflection away from the activated elements. Second, rotating or shifting a given pattern systematically alters the steering direction. For example, rotation of a circular-inspired pattern around the z-axis rotates the beam steering azimuth φ accordingly, while vertical displacement of the same distribution biases the main lobe toward the direction of the shift. This behavior highlights the geometric correspondence between the pattern orientation and the resulting beam direction. Third, the density of ON/OFF transitions within a pattern directly impacts the achievable steering angle. Patterns with more compact spacing between ON and OFF regions tend to produce stronger phase gradients across the RIS, resulting in larger steering angles (up to θ ≈ 36° in this work). Similar effects have been reported in [3], [7], where 1-bit reflectarray designs demonstrated enhanced steering performance but also increased side-lobe levels due to phase quantization and abrupt conductivity transitions. However, these more aggressive designs may also lead to degraded radiation quality, such as higher side-lobe levels or even beam splitting. Finally, a pronounced influence is observed when ON/OFF changes occur near the physical edges of the RIS. Modifying the conductivity at the array boundaries significantly alters the effective aperture distribution, leading to stronger steering effects compared to similar modifications in the central region. This edge sensitivity is consistent with aperture field theory, where discontinuities at the boundaries dominate the far-field radiation pattern [7]. Overall, these observations indicate that by systematically generating and testing ON/OFF distributions according to rules of translation, rotation, element spacing, and edge placement, it is possible to derive an empirical framework that predicts the steering performance of the RIS without requiring full-wave simulation for every configuration. Such empirical guidelines can greatly simplify the design and control of modest-sized binary RIS implementations. It is worth noting that deriving such empirical rules requires the systematic generation and analysis of a large number of distinct patterns. By studying how different geometrical arrangements affect the main lobe direction, a more comprehensive understanding of the RIS behavior can be achieved. This, in turn, enables the selection or design of appropriate patterns to steer the reflected beam toward any desired direction with a high degree of confidence IV. CONCLUSION This work presented the design of a simplified 1-bit RIS operating at 28 GHz. The RIS, composed of an 8×8 array of unit cells based on a compact diode-switched patch design, was analyzed under normal-incidence horn antenna illumination using full-wave simulations in ANSYS HFSS. By systematically applying a variety of ON/OFF control patterns, the resulting far-field radiation characteristics were extracted and compared. The results demonstrated that the spatial geometry of the applied binary patterns critically determines the steering behavior of the reflected beam. While uniform ON or OFF states resulted in broadside reflections, linear distributions introduced small angular deviations, and more complex or asymmetric configurations enabled steering angles of up to θ = 36°. Certain patterns, however, also exhibited beam splitting and elevated side-lobe levels, highlighting the inherent tradeoffs between steering range and main-lobe integrity. Overall, the findings of this study confirm that even modest-sized RIS arrays can achieve effective and controllable beam steering through structured binary modulation. Furthermore, the observed behavior provides valuable empirical insights into the relationship between pattern geometry and beam direction, forming the basis for low-complexity design strategies in practical RIS implementations. Future extensions of this work may consider larger RIS dimensions, experimental validation with fabricated prototypes, and the application of optimization algorithms for systematic pattern synthesis. ACKNOWLEDGMENT This research was supported by the European Union, through the Horizon Europe Marie Skłodowska-Curie Staff Exchanges Programme “6G intelligent connectivity and interaction for users and infrastructures (6G-ICARUS)” under Grant 101131342. The authors would like also to acknowledge the support provided by the Digital Governance Unit of the Aristotle University of Thessaloniki (AUTh) throughout the progress of this research work. REFERENCES [1] Y. Liu, X. Liu, X. Mu, T. Hou, J. Xu, and X. Tao, “Reconfigurable Intelligent Surfaces: Principles and Opportunities,” IEEE Communications Surveys & Tutorials, vol. 23, no. 3, pp. 1546–1577, 2021. [2] M. ElMossallamy, H. Zhang, L. Song, K. G. Seddik, Z. Han, and G. Y. Li, “Reconfigurable Intelligent Surfaces for Wireless Communications,” IEEE Transactions on Cognitive Communications and Networking, vol. 6, no. 4, pp. 990–1002, Dec. 2020. [3] H. Yang, F. Yang, S. Xu, M. Li, X. Cao, and J. 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