micromachines Article Rapid Prototyping of Bio-Inspired Dielectric Resonator Antennas for Sub-6 GHz Applications Valeria Marrocco 1,* , Vito Basile 1 , Ilaria Marasco 2,3 , Giovanni Niro 2,3, Luigi Melchiorre 2 , Antonella D’Orazio 2, Marco Grande 2 and Irene Fassi 4 1 STIIMA CNR, Institute of Intelligent Industrial Technologies and Systems for Advanced Manufacturing, National Research Council, Via P. Lembo, 38/F, 70124 Bari, Italy; [email protected] 2 Politecnico di Bari, Dipartimento di Ingegneria Elettrica e dell’Informazione, Via E. Orabona 4, 70125 Bari, Italy; [email protected] (I.M.); [email protected] (G.N.); [email protected] (L.M.); [email protected] (A.D.); [email protected] (M.G.) 3 Center for Biomolecular Nanotechnolgies, Istituto Italiano di Tecnologia (IIT), Via E. Barsanti 14, 73010 Arnesano, Italy 4 STIIMA CNR, Institute of Intelligent Industrial Technologies and Systems for Advanced Manufacturing, National Research Council, Via A. Corti, 12, 20133 Milan, Italy; [email protected] * Correspondence: [email protected] Abstract: Bio-inspired Dielectric Resonator Antennas (DRAs) are engaging more and more attention from the scientific community due to their exceptional wideband characteristic, which is especially desirable for the implementation of 5G communications. Nonetheless, since these antennas exhibit peculiar geometries in their micro-features, high dimensional accuracy must be accomplished via the selection of the most suitable fabrication process. In this study, the challenges to the manufacturing process presented by the wideband Spiral shell Dielectric Resonator Antenna (SsDRA), based on Citation: Marrocco, V.; Basile, V.; the Gielis superformula, are addressed. Three prototypes, made of three different photopolymer Marasco, I.; Niro, G.; Melchiorre, L.; resins, were manufactured by bottom-up micro-Stereolithography (SLA). This process allows to cope D’Orazio, A.; Grande, M.; Fassi, I. with SsDRA’s fabrication criticalities, especially concerning the wavy features characterizing the thin Rapid Prototyping of Bio-Inspired spiral surface and the micro-features located in close proximity to the spiral origin. The assembly of Dielectric Resonator Antennas for the SsDRAs with a ground plane and feed probe was also accurately managed in order to guarantee Sub-6 GHz Applications. reliable and repeatable measurements. The scattering parameter S11 trends were then measured Micromachines 2021, 12, 1046. https:// by means of a Vector Network Analyzer, while the realized gains and 3D radiation diagrams were doi.org/10.3390/mi12091046 measured in the anechoic chamber. The experimental results show that all SsDRAs display relevant − Academic Editor: Paulo M. Mendes wideband behavior of 2 GHz at 10 dB in the sub-6 GHz range. Received: 26 July 2021 Keywords: stereolithography; dielectric resonator antennas; Gielis superformula; wideband; sub-6 Accepted: 26 August 2021 GHz applications Published: 29 August 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional affil- Bio-inspired designs have proved to be effective in the enhancement of antenna iations. performance, especially for microstrip-based ones at higher frequency ranges, such as WLAN, WiMAX, 5G, and mm-wave [1]. A variety of natural shapes and geometries, showing improved bandwidth with reduced sizes and good radiation efficiency, has been widely discussed in research. In particular, flower-shaped patches conceived on a defective Copyright: © 2021 by the authors. ground structure (DGS) [2], Oxalis triangularis plant-leaf shapes for the realization of two Licensee MDPI, Basel, Switzerland. frequency-selective surfaces (FSS) [3], Carica Papaya leaf-based patches [4], sunflower- This article is an open access article seed-shaped surfaces [5], and fractal and self-similar fractals [6–9] have demonstrated their distributed under the terms and potential to achieve large bandwidth, high gain, multiband, and highly directive characteris- conditions of the Creative Commons tics. Additionally, a four-leaf clover geometry was exploited to realize patch elements of an Attribution (CC BY) license (https:// antenna array designed for wireless applications [10]. Investment in such nature-inspired creativecommons.org/licenses/by/ shapes has been further promoted by the introduction of Gielis Superformula [11,12], which 4.0/). Micromachines 2021, 12, 1046. https://doi.org/10.3390/mi12091046 https://www.mdpi.com/journal/micromachines Micromachines 2021, 12, 1046 2 of 15 facilitates the design of patch antennas [13–15], metamaterials and metasurfaces [16,17], frequency-selective surfaces (FSS) [18], and split-ring resonators [19,20]. Moreover, besides planar antennas, one of the most recent and intriguing uses of the superformula concerns the design of Dielectric Resonator Antennas (DRAs) [21,22]. Currently, DRAs are proposed as an interesting alternative to planar antennas for 5G and mm-wave applications, due to their ability to satisfy the demand of wide bandwidth and to overcome the issue of the conduction losses experienced by metal-based patch antennas at higher frequencies. In particular, DRAs made of low dielectric constant materials with low losses have proven capable of fulfilling this requirement better than metal-based antennas. Additionally, since polymers can be exploited for DRA fabrication, the opportunity to use Additive Manufacturing (AM) technologies is further increasing the interest in and efforts towards DRA optimization among the scientific community. As a matter of fact, several dielectric materials are suitable for use in AM technologies, although few of them have actually been evaluated and characterized in the microwave range. Among polymers, the thermoplastic Poly-Lactic Acid (PLA) [23–25] and, more recently, photopolymer resins [26], have been analyzed, characterized, and utilized. Nonetheless, even though low dielectric constant materials such as polymers can en- sure DRA wideband behavior, they show drawbacks related to antenna gain and efficiency. For this reason, the use of such materials has implied the exploitation of complex 2.5D and 3D geometries for the design of DRAs. To this end, some authors proposed a 3D quadrupole-shaped DRA suitable for WLAN (8 GHz) [24] and 5G applications (3.5 GHz) [25], and super-shaped DRAs (S-DRAs), based on the Gielis superformula, operating at 3.5 GHz [26–28] and between 6–13 GHz [29], respectively. Basile et al. [26] reported the design and manufacture of a star-based S- DRA and 3D twisted versions of this design operating at 3.5 GHz: the antennas were manufactured by micro-inverted stereolithography (SLA) and realized by means of a photopolymer resin, which was opportunely characterized in the frequency range of interest. The experimental results showed that this kind of S-DRA was able to provide a gain around 3 dBi and a radiation efficiency of ~90% at 3.5 GHz, with a significant reduction of the antenna volume and weight. Simeoni et al. [28] reported a classic 2.5D star-shaped S-DRA antenna made of polyvinyl chloride (PVC)—a thermoplastic material with low permittivity ("r ≈ 2.8) and negligible tangent loss—fabricated by means of the extrusion process. The antenna exhibited a fractional bandwidth of 74% and a gain of ~8 dBi between 6 and 13 GHz. Recently, spiral-based geometries were also considered for DRAs. Indeed, spiral and Fibonacci-based shapes have been frequently exploited in the antenna scenario to tailor planar and metal-based antenna performance. Fibonacci spirals were used for FSS implementation in microwave absorbers [30] and radome applications [31], or applied to modified Koch curves for antenna miniaturization [32]. Furthermore, Logarithmic spiral slots have been used for the implementation of cylindrical DRA multimode operations [33]. In addition, the design of a Spiral shell DRA (SsDRA), resembling the shell of a sea mollusk, has been presented [34]: the numerical results showed that the optimized antenna was capable of wideband behavior (up to more than 2 GHz, between 3 GHz and 6 GHz), with satisfactory gain and efficiency. Recently, an example of a one-round spiral shell realization was proposed [35]. Due to its small size (height of 12.5 mm and thickness of 3.5 mm), the spiral shell was manufactured by means of SLA. Finally, this element was filled with water and then mounted on a cylindrical DRA to generate multimode orbital angular momentum (OAM) waves at frequencies between 4.8 and 6.36 GHz. However, besides the aforementioned studies, the fabrication of full spiral-shape- based DRAs is still unaddressed by research, which is reasonable because of the complexity of their manufacture. In the present paper, the fabrication and the characterization of the optimized SsDRA design presented in by Melchiorre et al. [34] are reported: in particular, three prototypes were manufactured by exploiting different photopolymer resins. In Section2, the model- Micromachines 2021, 12, 1046 3 of 15 ing of the antenna geometry is shown. The challenges and criticalities exhibited by the SsDRA’s fabrication are described and discussed in Section 2.1, along with the details of the prototype modeling and manufacture, carried out by means of bottom-up micro-SLA. The assembly of the SsDRA was performed to ensure reliable and repeatable measurements: in this viewpoint, the required positioning accuracy, the mechanical fixation on the ground plane, and the correct feed probe