Matthew W. Nichols, Alfredo Gonzalez, Elias A. Alwan, John L. Volakis
IMAGE LICENSED BY INGRAM PUBLISHING
Combining antenna arrays with physical reconfigurability (i.e., origami) allows for additional degrees of freedom in operation and enables larger structures to be folded into smaller volumes. The packaging ability of origami antennas is of great interest for CubeSat applications in particular. However, traditional origami is based on the manipulation of thin sheets, making physical reconfigurability with thick substrates very difficult. In this article, we present a foldable series-fed patch array designed on a strictly rigid printed circuit board (PCB). Notably, the array does not require any external mechanical folding appendages (i.e., brackets, tapes, or offsets), and it can be manufactured using commercially available PCBs and equipment. The PCB itself is strategically cut to form lamina emergent torsion (LET) joints. When fully deployed, the array operates at 5.7 GHz. A significant shift is not seen in the operational frequency until the folding exceeds 60° (5.7–5.61 GHz). This shows that the array, even in a state of near extreme faulty deployment, will operate as intended. An 8 × 6 prototype array was fabricated using a Rogers DiClad 880. When fully deployed, the array is extended in a surface area of 280 × 198 × 1.524 mm and can be folded into a 35.5 × 198 × 12.5-mm compartment. The array held its integrity well after 100 cycles. (One cycle starts at a state of full deployment, moves to a state of being fully packaged, and goes back to a state of full deployment.) Measurements show that the active reflection coefficients are in good agreement with the finite array simulations. The measured realized gains at 5.7 GHz for 0° and 30° folds were 22.4 and 21.4 dBi, respectively. At the 60° fold, a realized gain of 20.6 dBi was achieved at the frequency of 5.61 GHz.
Satellite communications have recently shifted to a new paradigm with the adoption of CubeSats: using the trend of modern electronics, fully functioning satellites can be compressed into much smaller volumes [1]. Communication systems are no exception, but sub-6-GHz antennas can be cumbersome for these compartments. In other words, the development of origami electromagnetic (EM) structures is still hindered by several challenges, among them 1) a lack of approaches for reconfigurable and multifunctional EM origami structures, 2) a lack of design algorithms for simultaneous EM physics and origami math analysis, and 3) a lack of materials and mechanisms for robust and repeatable actuation. Here, we focus on the lack of materials and mechanisms for robust and repeatable actuation. Notably, there is a strong recent interest in folding and packaging large apertures on rigid substrates into small footprints [2]. An example can be seen in parabolic antennas, formed of wire meshes [3], [4]. To form the adaptability that mesh parabolics lack, further work has been conducted concerning reflectarrays [5], [6]. However, the challenge for materials and mechanisms for robust and repeatable actuation remains. Origami-folding techniques provide alternative approaches for these antenna arrays [7], [8], [9]. Attempts to realize origami-based structures using rigid PCB foldings have been reported in [10] and [11], but extensive testing in terms of radio-frequency (RF) performance is still limited. Notably, although flexible materials exist, their reliability is still an issue [7], [8], [9], [10], [11]. Specifically, when these substrates are exposed to harsh environments (extreme rain, wind, and temperature variations), they may be susceptible to damage.
In this article, we propose a foldable series-fed patch array, implemented on rigid homogeneous PCB, as depicted in Figure 1. Notably, the array achieves folding by strategically cutting LET joints in between the active antenna elements and requires no external folding appendages. Full-wave simulations are conducted (utilizing Ansys HFSS software) to analyze three states of deployment, namely, 1) case 1: a 0° fold where the array is fully deployed, 2) case 2: a 30° fold where the array is partially deployed, and 3) case 3: a 60° fold where the array is at a state of faulty deployment. A prototype is fabricated and measured to verify simulations.
Figure 1. The fabricated prototype. (a) Case 1: a 0° fold. (b) Case 3: a 60° fold. (c) Case 1: a 0° fold showing the 50-Ω coax feeding.
A foldable series-fed patch array was designed on a 1.5-mm-thick Rogers DiClad 880 PCB, intended to operate at 5.7 GHz when fully deployed. The array comprises eight rows, each consisting of six individual series-fed patches. Each individual patch is 18 × 20.4 mm, as depicted in Figure 2. Each of the eight rows, as a whole, will be referred to as a single element and fed at one end. The spacing along the plane of feeding was initially set at half of a wavelength (at 5.7 GHz) and optimized to reduce coupling. The transmission line connecting the patches is matched to 50 Ω. Also, the adjacent series-fed patch rows were placed approximately 0.52 X away and then shifted further for the addition of the LET Joints [11]. These spacings were optimized with the goal of keeping the array’s performance. The simulation model of the array can be seen in Figure 3.
Figure 2. A simulation model of the accordion-fold series-fed patch array using LET joints: the dimensions of a single periodic element.
Figure 3. The setup of the finite array simulations at different fold angles.
LET joints are cut directly into the PCB, requiring no external hinges, connections, or substrate add-ons. Additionally, they do not require any extra assembly and are part of the PCB fabrication. Initial prototypes were fabricated using the Rogers 5880, Rogers DiClad 880, Rogers 3003, Rogers 4350, Rogers 3010, and FR4. It was found that the Rogers DiClad 880 proved to yield the highest range of motion, with the greatest durability over a set number of complete folds (shown in Figure 4). Using the Rogers DiClad 880, periodic notches were cut out between the elements of the antenna array in 1D measuring 2 × 10 mm, with 5-mm horizontal spacing and 2-mm vertical spacing. These are depicted in Figure 4. Furthermore, the joints are carefully positioned and optimized to not only avoid substrate splintering during folding but also to minimize performance variations. We note that the rectangular cutouts were made with dimensions significantly smaller than the operational wavelength to prevent effects from the perforated ground plane. It is remarked that the fabricated prototype endured well more than 100 cycles.
Figure 4. The placement and dimensions of the LET joint arrays in the simulation model.
A finite 8 × 6 array was designed, modeled, and simulated. The finite array was evaluated under three different cases in deployment: 1) case 1: a 0° fold when fully deployed, 2) case 2: a 30° fold when partially deployed, and 3) case 3: a 60° fold at a state of faulty deployment. These simulations were intended as a reference for measurements. Simulations for the three folding cases gave the following gain results:
This analysis can be seen in Figure 5.
Figure 5. Simulations of a single center element impedance showing the growing mismatch as the array folds after the start of a 45° folding angle. This mismatch is due to the increased element-to-element coupling and is the cause of the frequency shift.
An 8 × 6 prototype of the accordion-fold series patch array was fabricated on Rogers DiClad 880 substrate. The active S-parameter measurements were conducted using the Keysight power network analyzer N5222B. The gain and voltage standing-wave ratio results are as follows:
These details can be seen in Figures 6 and 7. It is noted that only the four center element results are displayed in Figure 6, but all elements were in agreement with simulations. Gain measurements were achieved with a one-to-eight power divider in an anechoic antenna chamber at Florida International University. The power divider was de-embedded prior to plotting the results seen in Figure 7.
Figure 6. (a) The fabricated prototype, followed by (b)–(d) active matching measurements for the three analyzed states of deployment. Here, center elements 1–4 are fed, with the rest being terminated to 50 Ω.
Figure 7. (a) The measured versus simulated beam pattern of the 8 × 6 fabricated antenna array prototype at the three states of deployment. The measured gain corresponds to the excitation of all array elements using a one-to-eight power divider. The top row shows the H-plane, with the bottom row showing the E-plane. (b) The antenna array, coax cables, and power divider. This power divider was de-embedded from the plotted patterns.
A manual “move and hold” method was employed to achieve deployment and to keep the structure at certain fold angles. This method was chosen because the focus of this article is on LET joints and the folding approach. The mechanical actuation mechanisms have already been discussed elsewhere in [12], [13], and [14]. Notably, no appreciable performance degradation was seen after 100 folding cycles. (The array was folded and unfolded manually.) Furthermore, depending on the application, different feeding approaches can be used. For example, a microstrip-line power divider could be employed. In that case, the transmission lines could be placed over LET joints, as discussed in [10].
This article presented a folding technique for CubeSat arrays. It was demonstrated that both the array structure and its RF performance are maintained after repeated folding cycles. A proof-of-concept array was also fabricated to demonstrate performance showing an optimal gain at 5.7 GHz. Furthermore, the accordion-fold technique requires no additional appendages, thereby minimizing cost and assembly requirements. The designed LET joints have high levels of durability, with no signs of snapping or wear after 100 folding cycles. A change in the intended frequency operation is seen at a folding of 60° or greater.
This work was supported in part by the Air Force Office of Scientific Research Grant FA9550-19–0290 and by NASA Grant 80NSSC18K1736.
Matthew W. Nichols (mnich036@fiu.edu) is currently pursuing his Ph.D. degree in electrical engineering with the RFCOM Lab, Florida International University, Miami, FL 33174 USA. He is a Student Member of IEEE.
Alfredo Gonzalez (agonz341@fiu.edu) is currently pursuing his M.S. degree in computer science at the Georgia Technical Institute of Technology, Atlanta, GA 30332 USA. He received his M.S. degree in electrical engineering from Florida International University, Miami, FL 33174 USA. He is a Member of IEEE.
Elias A. Alwan (ealwan@fiu.edu) is an Eminent Scholar Chaired Assistant Professor with the Department of Electrical and Computer Engineering, Florida International University, Miami, FL 33174 USA. He is a Member of IEEE.
John L. Volakis (jvolakis@fiu.edu) is the dean of and a professor with the College of Engineering and Computing, Florida International University, Miami, FL 33174 USA. He is a Life Fellow of IEEE.
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Digital Object Identifier 10.1109/MAP.2022.3229295