Electromechanically Deployable High-Gain Pop-Up Antenna using Shape Memory Alloy and Kirigami Technology

This paper proposes a low-cost, high-gain, and vertically polarized deployable antenna utilizing kirigami pop-up geometry. It primarily utilizes a foldable polyethylene terephthalate sheet to produce kirigami geometry in association with a rectangular radiating monopole, two reflectors, and a parasitic strip director. The reflectors and director increase the antenna gain and provide a frequency-independent tilted radiated beam with a higher beamwidth in the azimuth plane. In addition, electromechanically excited shape memory alloy (SMA) actuators enable folding and unfolding, make the antenna easily transportable and swiftly deployable. We describe the step-by-step fabrication of the kirigami geometry and shape memory spring actuator characterization. The designed, fabricated, and tested antenna achieves a -10 dB reflection bandwidth of 48.8% (1.7–2.8 GHz) providing a peak gain of more than 10 dBi at 2.45 GHz. The tilted radiated beam has a significantly wider beamwidth (90°) in the azimuth plane, compared to 40° in the elevation plane. The measured results agree well with simulations, verifying the proposed design concept. The fabricated prototype offers cost-effectiveness, more rapid fabrication, unprecedented performance, and significant potential for use in a range of microwave applications.


I. INTRODUCTION
The rapid growth in military communication systems has led to the demand for new standards of deployable antennas with high gain, a tilted beam, and a wide beamwidth. Lightweight, high-gain, and deployable antennas that operate in the microwave frequency range are particularly suited for military communication because conventional satellite antennas are quite bulky, requiring vehicular or helicopter transport [1]. In general, a tilted-beam antenna with high-gain characteristics can be created by fabricating an array of multiple radiating elements and electronically adjusting their phase at the input ports [2][3][4][5][6][7]. A number of directive high-gain antennas with tilted beams based on different technologies have been reported [5,[8][9][10], but lightweight, high-gain origami/kirigami antennas represent a particularly attractive option because these folded antennas can easily be carried by military personnel and subsequently unfolded and deployed in the desired location [1].
Origami, which was first developed in Japan, involves the design of various crafts based on the folding and unfolding of paper sheets. Origami concepts have recently been employed in various interesting applications, including mechanical metamaterials [11], flexible electronics [12], soft folding robots [13], pneumatic actuators [14] and several others [15][16][17]. It offers a number of advantages, such as a lightweight design, easier deployment, and easier and faster fabrication. The benefits of origami in practical applications have recently been demonstrated with the fabrication of highgain antennas. For example, a high-gain, tilted-beam, deployable origami antenna [18] using tetrahedron geometry achieved a gain of more than 9 dBi with a 60° tilted beam. However, the direction of the main beam changed at different frequencies due to the lossy paper substrate, and transportation was challenging due to the bulky dimensions of the antenna. A high-gain deployable antenna using origami magic spiral cubes achieved a peak gain of 7.3 dBi, but fabrication was time consuming due to the construction including 3 magic cubes [19]. Another deployable origami antenna [1], [20] was designed for military applications, with a 60° tilted main beam. However, a gain of only 5.7 dBi was achieved with a significant side lobe level (1 dB) and a narrow beamwidth (60°). In addition, an origami-inspired deployable 2×2 corporate-fed microstrip patch antenna array [21] that could be easily reconfigured using a Miura-ori fold pattern was fabricated and achieved a gain of 5 dBi. Despite these advances, some earlier origami antennas suffered from low efficiency, limited flexibility, and spatial complexity. For example, the origami antennas reported in [1,18,20], were intended to be carried as paper sheets and fabricated when needed; however, fabrication was time-consuming and the final antennas were not robust. Kirigami is an alternative form of origami that includes paper cutting and represents a notable advancement in origami technology. It retains all of the original benefits of the origami technology and incorporates additional features, including a greater degree of freedom in terms of flexibility and the final shape. Origami is mainly based on folding and unfolding techniques, whereas kirigami geometries are constructed by making cuts in the planar substrate sheets before folding. Consequently, the sheets can be transformed into complex three-dimensional (3D) patterns producing compression and tension [22][23][24][25][26][27][28]. A desired shape can be realized by making positional-dependent cuts in specific patterns on a flexible substrate to enhance reconfigurability via stretching, twisting, and bending. Thus, kirigami technology has been extended from paper substrates to the use of more robust and/or flexible substrates. This approach has recently received significant attention because of its use in deployable, foldable, stretchable, transformable, and reconfigurable electronics [26,27]. For example, several studies have demonstrated lithium-ion batteries and supercapacitors fabricated using kirigami technology [23,24,27], and it has also been useful for the development of biomedical strain sensors [28]. Kirigami also opens new avenues for antenna design due to its various advantages, including its cost-effectiveness, lightweight nature, flexibility, convenient transportation, and rapid deployment. It not only has lower fabrication costs and a more rapid manufacturing process but can also be exploited for antenna geometries that are prohibitively difficult to achieve using conventional approaches [15][16][17].
In addition, to enhance the coverage range and field of view of tilted antennas, a wide beamwidth in the azimuth plane and a narrower beamwidth in the elevation plane are required. Because most users are close to the ground, power transmitted into the sky is wasted and should be reduced where possible [29]. However, most current tilted-beam, high-gain antennas suffer from a limited tilted bandwidth or a narrow beamwidth.
This paper reports a high-gain antenna with a wide beamwidth in the azimuth plane. Compared with conventional high-gain antennas, the proposed antenna is vertically polarized, which is less prone to path loss and hence more likely to achieve superior radiation performance and offers stronger security [30][31][32][33]. The vertical monopole configuration is realized by exploiting kirigami pop-up geometry. We employ a foldable polyethylene terephthalate (PET) sheet to form the kirigami geometry in association with a rectangular radiating monopole, two printed-circuit-board (PCB) reflectors, and a parasitic strip director. The reflectors and director increase the antenna gain and provide a frequency-independent tilted radiated beam with a higher beamwidth in the azimuth plane. The inclusion of shape memory alloy (SMA) actuators allows the antenna to be folded and unfolded, making it easily transportable and swiftly deployable. We optimize the kirigami fabrication process and the characteristics of the SMA spring actuators in designing the proposed prototype. The fabricated prototype offers costeffectiveness, more rapid fabrication, and unprecedented performance. It also exhibits significant potential for use in microwave applications related to military communication for tanks.
The remainder of this paper is organized as follows. Section II describes the design principles of the kirigami antenna, including the overall fabrication process and rigorous parametric analysis. Section III discusses the electromechanical actuator and its application in the proposed antenna for folding and unfolding. Section IV presents a detailed design analysis and validation of the proposed antenna by comparing simulation and experimental VOLUME XX, 2017 9 results. Finally, a conclusion is drawn in section V, discussing the advantages and limitations of the proposed antenna.

A. Kirigami Antenna Design
The proposed antenna comprises a rectangular monopole as the driven element with two reflectors and a director. We utilize kirigami geometry to design the antenna, with hard copper PCB sheets as the reflector elements. The pop-up geometry based on folding and unfolding ensures that the antenna is readily deployable. Figure 2 shows the individual design steps for the proposed kirigami geometry. We use a PET sheet (145 mm × 145 mm, 0.25 mm thick) for physical robustness. The kirigami pattern is then printed onto the PET substrate, as shown in Fig. 2 2c). All of the horizontal segments are sequentially folded starting from lower horizontal segments (Fig. 2d), and the unfolded kirigami design produces a 3D staircase shape (Fig. 2e). The kirigami geometry consists of seven steps, with the driven monopole on the third and the director on the fourth step; hence, the director is 6 mm higher than the driven monopole (Fig. 2a). The Kirigami structure allows it to be folded and unfolded; Fig. 2(f) shows it in its folded state. Two copper PCB sheets are attached to the back of the kirigami geometry to act as reflectors and support for the antenna to ensure it is sufficiently robust. Folding and unfolding are realized using SMA springs that can be expanded and compressed/squeezed under the user's control to provide electronic tunability. Figs. 2(g) and (h) show the assembled antenna in its unfolded (i.e., operational) and folded states, respectively. The SMA actuators attached to the back of the geometry allow reflector #2 to be unfolded to 15° from the Z-axis. The antenna is excited using a 50-Ω subminiature version A connector with the inner pin connected to the rectangular monopole (Fig. 1) and the ground connected to the reflector. We perform full-wave simulation for the proposed antenna using the ANSYS High-Frequency Structure Simulator (HFSS), with a dielectric constant and dielectric loss tan δ of 3 and 0.02, respectively for the PET sheet [34], and a copper film conductivity of 4.4×10 5 S/m [19]. Although this is lower than typical copper conductivity (5.96×10 7 S/m) [35], it is sufficient to ensure reasonable efficiency [16].

B. Antenna Element Effects on the Radiation Patterns
Several antenna element combinations are considered to realize the desired tilted radiation pattern and beamwidth. Fig.  3(a) shows the simulated 3D radiation pattern for a monopole antenna without a reflector or director. The antenna exhibits an omnidirectional pattern with a gain of only -0.1 dBi. We include reflector #1 to provide directionality and to increase the gain. Figure 3(b) shows the corresponding simulated 3D radiation pattern with a gain of 4.5 dBi. Reflector #2 is then added to provide a 60° tilted radiation pattern (Fig. 3c), subsequently enhancing the gain to 9.6 dBi. Finally, we place a 14-mm long rectangular strip director from the driven element to increase the gain in the direction of the tilted beam. The final antenna design achieves a peak gain of 10.55 dBi with a tilted beam direction of 60° in the elevation plane (Fig. 3d).

C. Parametric Analysis
We conduct parametric analysis to study the effect of the design parameters on the antenna properties. In particular, we consider the monopole length (L m ), the distance between the rectangular monopole and reflector #2 (D 1 ), and the number of parasitic directors in seeking to optimize the resonance frequency with a suitable peak gain and beamwidth. Fig. 4(a) shows a decrease in frequency with increasing L m . Because the desired resonance frequency is 2.4 GHz, we select L m = 26 mm. The effect of D 1 on the antenna peak gain without any director can be observed in Fig. 4(b), where the peak gain increases to 9.6 dBi by increasing D 1 to 50 mm and subsequently decreases. Therefore, we select D 1 = 50 mm to achieve the highest peak gain with reflector #2. Figure 4(c) shows the simulated peak gain with respect to the number of directors. A single director increases the gain to 10.60 dBi from 9.60 dBi, but a second director reduces it to 9.58 dBi.
The proposed antenna geometry produced a wider beamwidth than conventional Yagi antennas, hence adding a second director did not increase the antenna gain. Therefore, we used a single director, achieving a peak gain of more than 10 dBi with a large beamwidth in the azimuth plane. We use SMA actuators to electronically fold and unfold the antenna geometry. Figure 4(d) compares the gain achieved with respect to the operating frequency with and without the SMA actuators. The actuators have a negligible effect on antenna performance because their length is much longer than the monopole antenna. Although reflector #2 can only be unfolded to 15°, simulations verify that this has no effect on the characteristics of antenna performance, including impedance matching, peak gain, and elevation beam direction.

A. Electromechanical Actuators
SMA springs are electromechanical actuators that can be deformed depending on the applied excitation voltage. The SMA actuators employed to fold and deploy the proposed kirigami geometry have a thin contraction-type helical structure with a strong temperature-dependent linear actuation without the need for moving parts. Applying a suitable voltage for a given period will restore the SMA shape and retain it until it is mechanically deformed at room temperature. This way, even though the spring is elongated at room temperature by applying an external force, passing DC through it will restore  its original length. The selected SMA spring can be extended to approximately 130 mm at room temperature, and it possesses a force of 5 N during contraction. This pulling force is imposed to fold and unfold the proposed antenna geometry by increasing the total spring length to more than 80% of the initial compressed state. Because an SMA spring has one-way memory, the proposed geometry employs springs on the front face (S f ) to fold the antenna geometry, and three springs are employed on the back (S b ) to unfold it. Therefore, this set of actuators is responsible for the folding and deployment of the antenna. The folded and unfolded state of reflector #2 (see Fig. 1) reflects the corresponding folding and deployment of the kirigami geometry, keeping reflector #1 stationary. In particular, applying approximately 2 V to S f compresses the springs to fold reflector #2, and elongate S b . Similarly, passing current through S b unfolds the kirigami geometry. SMA spring actuators with an extended length of 130 mm are required to fold and unfold the geometry. The selected SMA springs require 23 seconds to contract and are relatively insensitive to minor fluctuations in ambient temperature. SMA springs have been previously used in the design of frequencyand pattern-reconfigurable antennas [37,38]. It is important to note that SMA-based switching cannot compete with other tunable and switchable microwave devices in terms of speed. This slow response speed is attributed to inefficient heat transfer [39]. Recently, research on SMA actuators has increased substantially, particularly in terms of their actuation speed, which can be enhanced with quick heating. For example, a fast and energy-efficient actuation strategy has been proposed based on short pulses in the millisecond range to reduce actuation times to approximately 20 ms [39,40]. This approach can also reduce the energy used by up to 80% compared with conventional quasi-static actuation. Therefore, we expect that the low-switching speed of SMA actuators will be solved in the near future.

B. Shape Memory Alloy Actuator Characterization
In the spring characterization process, external DC excitation voltage ranging from 0.7 V to 2 V with a current of 3 A is applied to the open ends of the SMA springs (Fig. 5a). This provides a clear indication of the limits of their stretching ability. We define stretching as the total variation in the spring length divided by the time elapsed. Initial stretching is approximately 180 mm/s for a pitch of 9 mm, which is considered the fully stretched state. Figure 5(b) shows that, with the application of 2.58 W, the spring starts compressing and continues until 4.5 W, at which the stretching is only 4 mm/sec, negligible actuation is observed beyond this. Figure  5(c) presents the active spring region with respect to time. The spring acts very quickly for approximately 10 s. Actuation starts after 3 s and the spring pitch decreases until 15 s. The spring demonstrates almost no subsequent actuation with an input power of 4.5 W. Figure 5(d) displays the results of the test for SMA spring repeatability, in which the selected SMA spring is compressed multiple times from its fully expanded state (L e = 130 mm) using the applied voltage.
A maximum deviation of 2.5 mm is observed from the initial compressed state (L c = 25 mm) after 25 trials, which alters the tilted beam angle of the proposed antenna by only 2°. Hence, the proposed SMA-based beam-reconfigurable antenna shows acceptably stable performance after repeated use.

IV. Experimental Verification of Antenna Performance
The proposed deployable kirigami antenna is fabricated on a PET sheet (Fig. 2). The S-parameter of the antenna is measured using an Anritsu MS2038C vector network analyzer, and a comparison of the simulated and measured reflection coefficients are presented in Fig. 6. The simulated and measured reflection coefficients are in good agreement, covering a frequency range of 1.7-2.8 GHz, with a -10 dB impedance bandwidth of 48.8%.
The minor difference between the simulated and measured reflection coefficients is due to folding errors and fabrication tolerance. The 3D radiation pattern of the antenna is measured in a shielded commercial radio frequency anechoic chamber. The simulated and measured 2D radiation patterns of the antenna in the elevation plane are presented for frequencies of 1.8, 2.1, and 2.4 GHz in Fig. 7.
The main beam of the antenna is directed towards θ of 60° for all frequencies. The two reflectors and the director are employed to achieve the desired tilted beam. Figure 8 shows that the measured normalized radiation patterns in the azimuth plane achieve a beamwidth of more than 90°. The combination of the two reflectors and the director also helps to achieve a wide beamwidth in the azimuth plane. The measured 3D radiation patterns for the antenna at frequencies of 1.8, 2.1, and 2.4 GHz are depicted in Fig. 9.    In Fig. 10(a), the simulated and measured peak gains are plotted for the -10 dB impedance bandwidth of the antenna. At 2.4 GHz, the measured peak gain of the antenna is 10.3 dBi, compared to 10.6 dBi for the simulation. Figure 10(b) shows the simulated and measured radiation efficiency for the proposed antenna. The measured radiation efficiency is calculated from the ratio of the measured peak gain to the simulated directivity, achieving 76.47% to 96% over the antenna's entire operating impedance bandwidth. This enhanced radiation efficiency is largely due to the lowloss PET sheet. As a result, the antenna efficiency is significantly higher than other previously reported paperbased origami antennas [18], [20], [41]. The proposed antenna also achieves a large beamwidth in the azimuth plane and offers swift mechanical deployment. In addition, the manufacturing process is much simpler, faster, and easier than previously reported deployable antennas [18], [20], [41]. Compared to conventional high-gain antennas, the proposed antenna is vertically polarized, which reduces the path loss, and vertical wire antennas generally achieve better practical radiation performance and have greater security [30][31][32][33]. The proposed antenna is low-cost due to its simple and easily obtained parts.

V. Conclusion
This paper presents a low-cost, lightweight, high-gain, vertically polarized deployable antenna based on a kirigami pop-up geometry. The antenna primarily utilizes a foldable PET sheet to form the kirigami geometry in association with a rectangular radiating monopole, two reflectors, and a parasitic strip director. Electromechanically excited SMA actuators allow the assembled antenna to be quickly stowed, ensuring easy transportability and swift deployment at the desired location. The fabricated prototype achieves a -10 dB reflection bandwidth of 48.8% (1.7-2.8 GHz), a peak gain of more than 10 dBi at 2.45 GHz, and a 60° tilted radiated beam with significantly larger beamwidth (90°) in the azimuth plane. The proposed antenna is suitable for tanks and other forms of military communication. Compared to existing conventional military field antennas, the proposed antenna is low-cost and offers very simple and rapid fabrication, making it suitable for numerous deployable antenna applications. Future deployable antennas can employ fast-switching SMA springs to further increase the deployment speed.