Multibeam Antenna for -Band CubeSat Connectivity Using 3-D Printed Lens and Antenna Array

In this letter, the design of a passive multibeam lens antenna is proposed for the CubeSat space communication system as an alternative application of a 2-D microstrip antenna array that has originally been designed for a 39-GHz 5G multi-user, multiple-input, multiple-output (MU-MIMO) system. The half-ellipsoid lens is 3-D printed using stereolithography technology. The antenna prototype is capable of selecting the main beam between 16 different directions with a gain ranging from 14 to 16 dBi and a half-power beamwidth of 14<inline-formula><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula>–18<inline-formula><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula>. The measurements carried out in an anechoic chamber show good agreement with numerical simulations. The presented prototype shows that by employing 3-D printing technologies existing antennas can be easily and inexpensively converted to switched-beam or multibeam solutions.

The wireless communication in the IoST can be realized in a wide spectrum, ranging from L-band (1-2 GHz), through X-band (8)(9)(10)(11)(12), up to Ka-band (26.5-40 GHz) [1]. The key components of wireless networks are antennas, which should provide the best possible performance preserving the smallest possible dimensions as the available space inside satellites is limited [4]- [6]. In the millimeter-wave bands, high-gain antennas are often needed. Usually requiring a large aperture, they are difficult or impossible to realize within the CubeSats' size constraints. Alternatively, a low-gain antenna element can be coupled with a dielectric lens, thus increasing the gain without an excessive growth of the physical aperture [11]. Adding a lens can also turn an antenna comprising a set of separately fed radiators into a multibeam or switched-beam antenna. Such antennas provide beam diversity that is often preferred in various IoST scenarios to improve connectivity. In the context of CubeSats, passive multibeam antennas [7] and single-input reconfigurable antennas, such as switched-beam antennas [8]- [10], are particularly interesting. Although they usually provide less flexibility than digital beamforming solutions, they have lower complexity and energy consumption.
In the literature, several examples of lens-based switchedbeam and multibeam antennas can be found [11]- [21]. The typical design consists of an antenna (or an array of planarly distributed antennas) placed behind a dielectric lens [11]- [18]. In [11], the general lens-based multibeam antenna and its implementation were presented. The beam direction can be deflected by changing the active radiating microstrip patch. Similar designs with added switched feeding networks are investigated in [12] and [13]. Designs in [19]- [21] show a similar approach to realizing switched-beam or multibeam antennas, where multiple antenna radiators are placed around a single lens. Each radiator is aligned with one of the lens' axes of symmetry and the beams are focused in those directions. This approach provides wider scanning angles than by placing radiators in one plane. A disadvantage of these solutions is that they are usually very large compared to the wavelength and would be difficult to fit into a small satellite.
The dielectric lens used in the switched-beam antenna designs can have ellipsoidal [11], [18], hemispherical [12]- [14], circular [19], [21], or even flat shapes [15]- [17]. Regardless of the shape, which can be chosen arbitrarily, the lens is usually optimized to maximize the gain or angular coverage of aggregated antenna beams. The antenna lenses can be quickly and affordably manufactured in 3-D printing technology, such as stereolithography (SLA) or fuse deposition modeling (FDM) [14], [17], [22]- [26]. In this letter, we propose a new approach different from the state-of-the-art designs, which optimize the system of radiators together with the lens to create an integrated switched-beam or multibeam antenna. In our work, an existing antenna array, designed, optimized, and fabricated for a standalone application in fifth-generation (5G) multi-user, multiple-input, multipleoutput (MU-MIMO) systems, is adapted to create a passive multibeam antenna intended for space applications by coupling it with a 3-D printed dielectric lens. Alternatively, the proposed antenna can be used for beam switching operation if an appropriate switching circuit is added. In the letter, we describe the employed antenna array, present the design and fabrication of the lens, and discuss measurement results comparing them with the numerical simulations. The presented modular approach, which is the novelty of this work, could be employed for any multiport planar antenna array designed for another mode of operation, such as digital or analog beamforming, particularly intended for intersatellite communication. It is especially beneficial in CubeSats, where lightweight printed lenses can be inexpensively customized to match particular mission communication scenarios without changing the radiator array and feeding network.

II. ANTENNA DESIGN
The proposed antenna design utilizes a previously realized and measured multiport linearly dual-polarized microstrip array [27], presented in Fig. 1. In this letter, only the horizontal polarization is investigated. The array is formed by 16 elements, each consisting of a radiating patch and feeding network. The microstrip patches are placed with a spacing R between their centers, which is constrained by connector size and a diameter of a single patch. The R value was chosen as 0.75 λ 0 , where λ 0 is free space wavelength at the resonant frequency (39 GHz). The design uses a two-layered substrate with patches on the top layer (CuClad217 substrate with 0.254 mm thickness) and a feeding network on the bottom layer (RT/duroid 5880 substrate with 0.127 mm thickness).
A circular microstrip patch of diameter D is chosen as a radiating element due to its symmetry required for dual-polarized operation [28], [29]. The patch, as shown in Fig. 1, is fed through two orthogonal C-shaped slots in the ground plane. The slots are defined by three parameters: width W s , lengths L s1 , L s2 , and are placed with a distance S 1 from the patch center. The antenna is fed through sub-miniature push-on, sub-micro (SMPS) connectors soldered to feeding microstrip lines on the bottom layer. They have a length of L 1 and are placed with an offset of S 2 from the patch center. The numerical values of the dimensions are presented in Table I.
The array was adapted to a design of multibeam antenna by adding a homogeneous all-dielectric half-ellipsoid shaped lens, providing both beam deflection and gain enhancement. 2-D beam selection of the antenna is realized by exciting ports of single patches selectively. This allows for an alternative beam-switching operation by using an optional switching circuit connected to the SMPS ports. Deflection of 16 separate beams is possible due to the diversity of patch positions relative to the axis of the lens. A side view of the antenna with the array printed circuit board (PCB) and SMPS connectors is presented in Fig. 2.
The lens was optimized by changing its dimensions (D L , H L , S L ) for fixed ε r = 2.66. The goal consisted of three criteria  regarding beam shaping: 1) scan loss lower than 3 dB, 2) 360 • coverage in the horizontal plane (variable φ) for a given θ, and 3) beams wide enough to provide continuous coverage with gain variation less than 6 dB. Dimensions of the designed lens are as follows: Diameter D L = 37 mm, which corresponds to 4.8λ 0 at 39 GHz, height H L = 16.5 mm, and the spacing S L = 5.5 mm. The air gap prevents severe degradation of the antenna matching and has an impact on the main beam direction. Using a lens with larger diameter increases antenna gain, but at the same time limits beam deflection angle. The maximum antenna radiator to lens center offset is 8.7 mm in both array axes.
The gain plots of all 16 beams obtained from simulations performed using Altair Feko 2021.2 software are shown in Fig. 3(a) and (b). Fig. 3(a) shows half-power gain coverage relative to the maximum gain for each beam. The beams are denoted by the symbols of corresponding excited patch radiators introduced in Fig. 4. An aggregated gain plot as a heat map with a maximum value equal to the common maximum gain of all beams G max = 16.9 dBi (for beam V A3 ) is in Fig. 3(b). The maximum simulated gain of individual beams ranges from 14.3 dBi to G max , whereas the maximum gain of a single-patch radiator is 7.4 dBi. The plot shows that the antenna provides continuous coverage in the whole 360 • range for φ and 0 • -33 • range for θ with the gain variation of mostly less than 5.2 dB. There are only a few very narrow spots where the gain falls from G max by 7.5 dB. From this plot, one can also read the directions of the beams and choose the angular cross-sections for corresponding radiation pattern measurements. Simulated antenna efficiency ranges from 48% to 53% and sidelobe level (SLL) in maximum gain cross-section ranges from −19.2 to −14.3 dB.

III. FABRICATION AND MEASUREMENTS
The designed antenna array was manufactured in a standard multilayer PCB technology and its top and bottom view with 3-D printed holder are presented in Fig. 5(a) and (b), respectively. The lens was fabricated using 3-D printing SLA technology, which is more precise compared to simpler 3-D printing methods such as FDM. FormLabs High Temp resin with dielectric properties reported in [31] is equal to ε r = 2.66 and tanδ = 0.03 was used as a dielectric material. Being durable and having a low density of 1.28 g/cm 3 , it is a competitive material to silicon glass used in [12] concerning the lens weight, which is relevant in space applications, particularly in CubeSats. The weight of the fabricated lens is 15.2 g. The fabricated prototype of the antenna is shown in Fig. 2. The lens is mounted at a fixed distance from the array PCB using a 3-D printed spacer (a green structure in Fig. 2).
All measurements were conducted using a measurement setup consisting of an anechoic chamber, an R&S ZVA50 network analyzer, a transmitting antenna, and an antenna under test in receiving mode, held by a stand placed on a turntable. Radiation patterns in the elevation plane are measured in the range of θ from −80 • to 80 • with a resolution of 2 • . Because the antenna array is uniform and symmetrical, only radiation patterns for a group of four representative radiating patches (V D1 -V D4 ) were measured while the rest can be approximated by mirroring. The vertical cuts were set individually for each beam at φ angles of  their maximum gain: Because of its broad beam, V D4 was also measured at φ = 290 • and 345 • for comparison with V D1 and V D2 . Fig. 5(c) presents the prototype in the anechoic chamber with marked directions of rotation for θ and φ angles.
The measurement results for beams V Dn are summarized in Table II. The simulated and measured normalized radiation patterns in the vertical cuts are compared in Fig. 6(a)-(c), and it can be noticed that they agree very well. The main lobe direction for all excited patches differs at a maximum of 2 • from simulation results. The most significant discrepancy is in SLL, which in the worst case increases from simulated −18 dB to measured −10.2 dB. The measured gain is equal to 6.5 dBi for the bare patch and in the range of 14.5-15.8 dBi for single beams (V D1 -V D4 ) with the lens. The 3 dB gain bandwidth is 12.3%. The average simulated dielectric loss in the lens is 2.3 dB at 39 GHz, which results in total simulated efficiency of 47% to 54% for the beams V D1 to V D4 . In effect, calculated directivity is 17.8-18.4 dBi, which corresponds to aperture efficiency of 26.2%-30.8%.
The measured values of the reflection coefficient for beams V Dn are shown in Fig. 7. There is only a small deterioration of the reflection coefficient caused by the lens so no further matching improvement is required. The −10 dB matching bandwidth for the worst case (V D3 ) is 38.3-40 GHz.

IV. CONCLUSION
In this letter, we present a design and experimental verification of a millimeter-wave multibeam lens antenna that directly adapts a separately designed multiport patch array by adding a 3-D printed lens. The antenna produces separate beams in 16 different directions and can be extended to provide a switched-beam operation. One of the advantages of the designed lens is that no additional optimization of the array is required because the designed lens has a minor influence on the antenna matching. Due to its small physical aperture and weight, the antenna is suitable for the IoST applications, such as intersatellite communication in CubeSat constellations. Another advantage of the proposed modular approach to designing lens-based multibeam antennas is that they can be customized for specific missions by replacing only inexpensive printed lenses.

ACKNOWLEDGMENT
This letter is a result of the BEYOND5 (www.beyond5.eu) project, which has received funding from the ECSEL Joint Undertaking (JU) under Grant 876124. The JU receives support from the European Union's Horizon 2020 research and innovation programme and from France, Germany, Turkey, Sweden, Belgium, Poland, Netherland, Israel, Switzerland, and Romania. The document reflects only the authors' view and the Commission is not responsible for any use that may be made of the information it contains.