# Low-profile Risley scanner using dielectric nonlocal metasurfaces for Ka-band SATCOM applications

T. Hoang, E. Vandelle, Ayoub Bellouch, Mahmoud Elsawy, Stéphane Lanteri,
B. Loiseaux

► To cite this version:

T. Hoang, E. Vandelle, Ayoub Bellouch, Mahmoud Elsawy, Stéphane Lanteri, et al. Low-profile Risley scanner using dielectric nonlocal metasurfaces for Ka-band SATCOM applications. EuCAP 2025 - 19th European Conference on Antennas and Propagation, Mar 2025, Stockholm, France. pp.1-5, 10.23919/EuCAP63536.2025.10999392. hal-05336354

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Submitted on 29 Oct 2025

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# Low-Profile Risley Scanner using Dielectric Nonlocal Metasurfaces for Ka-band SATCOM Applications

T. Q. V. Hoang*, E. Vandelle*, A. Bellouch†, M. Elsawy†, S. Lanteri†, B. Loiseaux*

*Thales Research and Technology, Palaiseau, France, van.hoang@thalesgroup.com

†University Côte d'Azur, INRIA, CNRS, LJAD, Sophia Antipolis, France, ayoub.bellouch@inria.fr

**Abstract**—2D beam steering is a crucial requirement for wireless communication systems in many modern and emerging applications that involve moving platforms. The mechanical scanning technique using two independently rotating deflectors, known as the Risley-prism concept, has emerged as a promising solution for cost-effective applications. Conventionally, both upper and lower deflectors are based on local metasurfaces, in which the deflection function is designed using a phase transformation generated by locally varying the dimensions of the patterns in each unit cell. In this contribution, a new design approach is applied by exploiting dielectric nonlocal metasurfaces, considering the interactions among adjacent subwavelength elements to shape the overall effective far-field response. Simulation results of a finite structure of 260 mm diameter, operating at Ka-band, are presented, demonstrating the potential of this new design approach.

**Index Terms**—2D-beam-steering, macrocell, nonlocal metasurface, optimization, satellite communication (SATCOM), terminal antennas.

## I. INTRODUCTION

Beam-steering antennas are essential components in many modern and emerging applications that involve moving platforms, such as satellite communication (SATCOM), aircraft and vehicle networks, and wireless millimeter-wave relays. Steering methods are typically categorized as mechanical, electrical, or hybrid. Electronically steered arrays enable compact, planar designs but have drawbacks such as significant power consumption and signal losses due to the active components. On the other hand, mechanical beam-steering antennas eliminate the need for electronic phase-shifting mechanisms, resulting in lower losses, reduced power consumption, cost savings, and simpler designs, though at the cost of slower steering speeds.

Recently, mechanical beam-steering antennas based on the Risley-prism concept have gained attention for their simplicity and effective beam-steering capabilities. The 2D-beam-steering systems based on the Risley concept typically consists of a single RF feed and two independently rotating prisms (Fig. 1). By adjusting the relative angles between the prisms, the beam can be steered in both elevation and azimuth. Considerable efforts have been made to enhance the performance of this concept. Amongst those, only a few designs incorporate dielectric metasurfaces for the deflectors, despite the fact that they can achieve high

transmission efficiency over a wide bandwidth (greater than 10%). Dielectric metasurfaces feature broadband operation due to the lack of resonance over wide frequency ranges and the straightforward implementation of matching layers. However, the drawback of all-dielectric metasurfaces is their thickness, which can reach one operating wavelength, compared to their planar printed counterparts whose thickness is much thinner (about several mm) but which exhibit a lower bandwidth. Recently, several works suggest optimization techniques to obtain improved performances of the Risley-based antennas. In [1], [2], the authors proposed a methodology to design and optimize the metasurfaces by considering the nature of the incident plane wave. They proved that a Risley-based system has better steering performance when the upper metasurface is optimized for oblique incidence, and the lower metasurface is optimized for normal incidence compared to when both upper and lower metasurfaces are optimized for normal incidence. However, their presented optimization strategy does not take into account different steering angles, and the performances are shown at only one frequency point. In [3], [4], the Pancharatnam-Berry phase technique has been exploited using identical planar printed elements, which are sequentially rotated to create a phase gradient that is able to deflect the impinging beam. The two deflectors have been optimized in order to scan a highly-directive beam with reduced level of the grating lobes. However, details in optimization strategy are not provided.

<figure><img src="image_2.png" /><figcaption>Fig. 1. 2D-beam-steering architecture based on the Risley concept.</figcaption></figure>

In our previous work, we introduced a low-profile Ka-band Risley-based steerable antenna, which features all-dielectric subwavelength deflectors combined with a large-aperture
RLSA antenna [5]. Two identical deflectors, designed using the phase gradient mechanism, are used for the steering system.

In this work, a new design approach is applied by exploiting dielectric nonlocal metasurfaces [6] for the subwavelength deflectors [7]. These deflectors will be separately optimized by a powerful Bayesian learning strategy [8], each deflector under a specific configuration of incidence angle, to obtain enhanced performance, both in transmission efficiency and axial ratio, over a wide frequency range and a wide scanning range. The novelty of this work is to optimize the upper deflector under an oblique incidence and at different azimuthal angles, representing different relative angles between the two deflectors, corresponding to different steering angles. This contribution will focus on the results of the final circularly-polarized Risley-based system operating in the frequency band from 27.5 to 31 GHz (Tx-Ka band for both civilian and military applications). The optimization algorithm and procedure will be detailed in another paper.

## II. MACROCELL DESIGN APPROACH

In the architecture shown in Fig. 1, the lower deflector receives a normally incident wave and transmits an output beam at an offset angle relative to the normal axis, which is 30° in our design. This 30°-incident-wave illuminates the upper deflector and forms the entire Risley-based system. Each deflector is a periodic structure composed of macrocells, with one dimension defined by the desired deflection angle and the other constrained by the subwavelength condition. In our design, the periodicity along x-axis is chosen to be 20 mm, corresponding to 2 times of wavelength at 30 GHz, to obtain a deflection angle of 30° at 30 GHz; while the periodicity along y-axis is chosen to be 2.6 mm, corresponding to about a quarter of wavelength at 30 GHz. This periodic arrangement generates grating lobes in the antenna radiation patterns, particularly when the beam is steered at high elevation angles, leading to reduced transmission efficiency. To achieve optimal steering performance, it is essential to strategically optimize the two deflectors.

The simulations of the macrocells are performed using Floquet analysis approach with periodic conditions, allowing the structure to be treated as if it were infinite. A left-handed circularly polarized (LHCP) plane wave is used as excitation. Each deflector's macrocell is composed of five cylindrical pillars, positioned at the center of the macrocell along y-axis but freely along x-axis, breaking the periodicity inside the macrocell. The pillar shape is used to improve the efficiency thanks to guiding effects [9]. The radius, the height and the position of the pillars are taken as parameters for optimization. Additionally, a matching layer with pyramidal shape is added above the pillars to improve the transmission efficiency. The height of these pyramids is also taken as optimization parameter.

The design approach is the following: firstly, the two deflectors' macrocells are separately optimized, each one under a specific configuration. The goal is to maximize the transmission efficiency of the co-polarization of the desired

diffraction order and minimize the axial ratio so that the circular polarization is maintained. While the lower deflector is optimized under normal incidence as shown in Fig. 2(a), the upper deflector is optimized under oblique incidence ($\theta_{in} = 30°$, corresponding to the output wavefront of the lower deflector) and with different azimuthal angle values representing the different relative angles between the two deflectors (Fig. 2(b) shows the configuration with azimuthal angle equal to 180°). The design presented in this paper was optimized to operate up to 47°-steering-angle, corresponding to a relative angle between two deflectors of 90°.

It is worth noting that the above simulation setup for the upper deflector assumes an ideal scenario in which the incident plane wave is perfectly circularly polarized. To study the actual performance, the entire macrocell composed of the two cascaded deflectors is simulated for different values of gap, the distance between the two deflectors, as shown in Fig. 2(c). In this configuration, the optimized lower deflector is rotated 180° with respect to the optimized upper deflector. The system of cascaded deflectors is excited with a normally incident plane wave in left-handed circular polarization.

<figure><img src="image_3.png" /><figcaption>Fig. 2. Simulation configuration of the macrocells: (a) lower deflector with normal input incidence, (b) upper deflector with oblique input incidence (30°) and with different azimuthal angle values, (c) two optimized deflectors in broadside radiation configuration in which the relative angle between the two deflectors is 180°.</figcaption></figure>

Fig. 3 presents the impact of the distance between the two deflectors on transmission efficiency and axial ratio. It seems that a distance of 5 mm (corresponding to a half of wavelength at 30 GHz) is the best tradeoff for efficiency and for axial ratio over the entire frequency range from 27.5 to 31 GHz. In the following section, we will use a 5mm-distance for the evaluation of the finite structure.
<figure><img src="image_4.png" /><figcaption>(a) Transmission efficiency of the co-polarization (LHCP component of the (0,0) order) versus frequency for different values of gap</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>(b) Axial Ratio (AR) versus frequency for different values of gap</figcaption></figure>

Fig. 3. Simulated results of the macrocell composed of two optimized deflectors in broadside radiation configuration in which the relative angle between the two deflectors is 180° - Impact of the distance between the two deflectors on transmission efficiency and axial ratio.

### III. NUMERICAL VALIDATION ON FINITE STRUCTURES

To evaluate the performance of the optimized deflectors in a more realistic scenario, finite structures with a total diameter of 260 mm are built. In this configuration, an LHCP Gaussian source is used as illumination. The structures are evaluated in CST Microwave Studio 2023 using the Finite Integration Technique (FIT) Transient Solver.

#### A. Gaussian source and one deflector

The evaluation is firstly carried out with only one deflector. The optimized lower deflector is compared with the conventional deflector, designed using the phase gradient approach with no optimization. The efficiency is calculated by using the directivity of the LHCP Gaussian source standalone, the directivity of the system composed of the LHCP Gaussian source and one deflector, and the cosine of the deflection angle (which is 30° at 30 GHz).

Fig. 4 depicts the performance of the finite structure, composed of an LHCP Gaussian source and one deflector, in terms of transmission efficiency of the LHCP co-polarization, Axial Ratio (AR) and Side Lobe Level (SLL) over the frequency range of interest. The optimized lower

deflector offers a very high efficiency exceeding 80% and a good circular polarization with AR less than 1 dB over the entire frequency range of interest from 27.5 to 31 GHz, demonstrating superior performance compared to its conventionally designed counterpart.

<figure><img src="image_6.png" /><figcaption>Fig. 4. Simulation results of the finite structure (260mm-diameter) composed of an LHCP Gaussian source and one deflector: either the optimized deflector Def. 1 (in solid curves), or the conventional deflector (presented in [5]) (in dashed curves).</figcaption></figure>

#### B. Gaussian source and two deflectors

Fig. 5 illustrates the full Risley-based steering system composed of the two optimized deflectors, illuminated by an LHCP Gaussian source.

<figure><img src="image_7.png" /><figcaption>Fig. 5. Simulation configuration of the finite structure (260mm-diameter) composed of an LHCP Gaussian source and two optimized deflectors.</figcaption></figure>

Fig. 6 presents the simulated radiation patterns in the main-beam azimuthal cut planes at 28 GHz and 29.5 GHz for four steering angles going from 0° to 47°, corresponding to four relative angles between the two deflectors $\Delta\phi = 180°$, 150°, 120°, 90°, respectively. A maximum directivity around 35 dBi is obtained at broadside. The beam is well-formed in all four configurations and the scanning loss follows the cos-law.
<figure><img src="image_8.png" /><figcaption>Fig. 6. Simulated radiation patterns for different steering angles at 28 GHz (top) and at 29.5 GHz (bottom), cosθ curve in black dashed line.</figcaption></figure>

Fig. 7(a) presents the transmission efficiency of the LHCP co-polarization component for different steering angles. This efficiency represents the amount of energy transmitted from the Gaussian source to a desired output direction, removing already the effect of effective surface on finite structures when the steering angle is different from 0°, so that one can compare the transmission efficiency at different steering angles. Besides, this efficiency considers the reflections (between the Gaussian source and the lower deflector, between the lower deflector and the upper one), the free-space loss between the two deflectors, the spillover loss when transmitting from the lower deflector to the upper one. As observed in Fig. 7(a), the system with the two optimized deflectors offers an efficiency higher than 50% for steering angles up to 47°, over almost the whole frequency range.

Fig. 7(b) presents the Axial Ratio over the frequency range of interest and shows that the system of two optimized deflectors offers an AR lower than 3 dB over almost the whole frequency range and for steering angles up to 47°.

Fig. 7(c) shows an SLL lower than -10 dB over the whole frequency range and for steering angles up to 47°.

It is worth noting that in the 180°-rotation configuration, one can compare the results obtained with the infinite structure (red curves in Fig. 3) and the ones obtained with the finite structure (red curves in Fig. 7). For the transmission efficiency, the only

difference is the spillover loss, which is not present in the infinite structure scenario.

<figure><img src="image_9.png" /><figcaption>(b) Axial Ratio versus frequency for different steering angles</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Fig. 7. Simulated results of the 260mm-diameter Risley-based system composed of a Gaussian CP source and the two optimized deflectors.</figcaption></figure>

## IV. CONCLUSION

For designing a Risley-based beam-steering antenna, a new design and optimization approach was applied. On the one hand, the use of optimized dielectric nonlocal metasurfaces has demonstrated superior performance compared to its conventionally designed counterpart, which is merely based
on phase gradient approach with no optimization. Simulation results of a finite structure composed of an LHCP Gaussian source and the optimized lower deflector have shown a very high efficiency exceeding 80% and a good circular polarization with AR less than 1 dB over the entire frequency range of interest from 27.5 to 31 GHz. On the other hand, the proposed optimization strategy, which takes into consideration the incidence angle and the different relative angles between the two deflectors, has proved its effectiveness in enhancing the performance of the full Risley-based steering system. Simulation results of a finite structure composed of an LHCP Gaussian source and two optimized deflectors have shown high transmission efficiency, low axial ratio and low SLL over a wide frequency range and a wide scanning range up to 47°. These promising results validate the proposed design and optimization methodology, paving the way for high-performance Risley-based steering antennas over a wide frequency range and a wide scanning range beyond the current version.

## ACKNOWLEDGMENT

This work has been partly funded by French MOD through the AEROCOM project (RAPID-2022290953). The authors would like to thank Philippe Pouliguen, Lucile Amory and Sylvain Gransart for their helpful technical discussions.

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