A multi-stable deployable quadrifilar helix antenna with radiation reconfigurability for disaster-prone areas

Domain: earth_science
Published: 2023-12-21
Editorial outcome: minor_revision

Abstract
In disaster-prone areas, damaged infrastructure requires impromptu communications leveraging lightweight and adaptive antennas. Accordingly, we introduce a bi-stable deployable quadrifilar helix antenna that passively reconfigures its radiation characteristics in terms of pattern and polarization. The proposed structure is composed of counter-rotating helical strips, connected by rotational joints to allow a simultaneous change in the helix height and radius. Each helical strip is composed of a fiber-reinforced composite material to achieve two stable deployed states that are self-locking. The reconfiguration between an almost omnidirectional pattern and a circularly polarized directive pattern enables the antenna to be suitable for both terrestrial and satellite communication within the L-band. More specifically, the presented design in infrastructure-less areas achieves satellite localization with directive circularly polarized waves and point-to-point terrestrial connectivity with an almost omnidirectional state. Hence, we present a portable, agile, and passively reconfigured antenna solution for low-infrastructure areas.

Introduction
Natural disasters devastate regions and eliminate available communication infrastructure, rendering rescue efforts extremely challenging1. Furthermore, underdeveloped regions already struggle with diminished infrastructure and have an underserved population that lacks basic communication needs, hindering humanitarian efforts. Antennas currently deployed in disaster-stricken regions consist of large dish reflectors with heavyweight and are not easily deployed or moved2–4. In addition, communication needs in regions with diminished infrastructure necessitate agility in antenna performance that enables terrestrial and satellite communications alike–a feature that existing solutions lack. Hence, there is a need to develop antenna systems that can deploy on demand and reconfigure their performance to satisfy arising circumstances while being lightweight and requiring minimum power.

Various types of deployable antennas have been investigated in literature including helical antennas, conical log spirals, quadrifilar helix antennas, origami and kirigami structures, and many other topologies5–14. While the deployable aspect of these antennas constitutes a major component of this work, the reconfiguration of these structures is another component that must be tackled. Reconfigurable antennas can switch their operational frequencies on demand, alter their radiation patterns, or achieve polarization reconfiguration15–20. Reconfiguration techniques range from the use of electrical switches21,22 to smart materials18,23,24 or to mechanical reconfiguration of conductors25,26. However, the merging of both deployment and reconfiguration mechanisms necessitates the use of an actuation system. Previous work has suggested traditional actuators such as motors or pneumatic actuators7,26,27 as well as smart material actuation18,19,23 that must be continuously powered to achieve a certain antenna topology. To address this challenge, the reconfiguration mechanism in this work is made completely passive by relying on a manual reconfiguration process, thus removing the need for dc power and external biasing. Such a feature constitutes a zero-power approach in implementing agile communications in diminished infrastructure areas. This work addresses the two challenges associated with a user-centric reconfigurable deployable antenna. First, it ensures that the antenna can be repeatably reconfigured into the desired geometries, which are all held passively. Second, coupling the passive geometries to the desired electromagnetic reconfiguration of the antenna itself.

Results
The proposed bi-stable deployable and reconfigurable quadrifilar helix antenna (QHA) for areas with diminished infrastructure is depicted in Fig. 1. The antenna28 deploys and offers two stable geometric configurations to achieve polarization and radiation pattern reconfiguration, leading to on-demand terrestrial and satellite communications. The stable geometries correspond to the minima of the stored strain energy in the structure. Therefore, the antenna will return to one of these two states automatically, even when perturbed away from them. Specifically, the antenna uses a helical lattice composed of counter-rotating helical strips connected by rotational joints allowing a simultaneous change in the helix height and radius. The strips are composed of a fiber-reinforced composite material whose anisotropic mechanical properties result in a lattice with two stable deployed states that are self-locking. The appropriate transition between the two states is ensured through a sliding feed topology. Such a feeding mechanism slides within the ground plane of the antenna and accompanies its geometrical deployment while maintaining the antenna operation within the L-band (1-1.2 GHz). The resulting structure with its structurally adaptive feeding is lightweight, with a stable frequency band response and reconfigurable radiation. In addition, the antenna is user-centric, and as a result, requires no Dc input power for reconfiguration while still ensuring repeatable shape change due to bi-stability. All these features define the major aspects of the proposed design and address the identified challenges of enabling agile communications in diminished infrastructure areas.

Reconfigurable fiber-reinforced polymer antenna
The proposed structure is composed of four conductive strips wound counter-clockwise and four dielectric strips wound clockwise. The strips are interconnected using non-conductive rotational joints as shown in Fig. 2a. The joints enable the whole structure to simultaneously vary its height, h, and radius, R. The reconfigurable deployable antenna concept proposed here is illustrated in Fig. 2b. The proposed design is based on a quadrifilar helix antenna where the dielectric strips act as a structural support for the conductive helices and enable reconfiguration between states. The deformation of the helical structure is enabled through the bending and twisting of the strips and any length changes are relatively small29. As a result, the height of the helix, h, is related to its radius, R, using (1)29, where l is the length for each of the strips and N is the number of turns.

h = 2\pi N \sqrt{\left( \frac{l}{2\pi N} \right)^2 - R^2} (1)

Reconfiguration is executed by applying a vertical force on the tips of the strips that make up the structure as depicted in Fig. 2b. This force enables the eight strips to slide along the structure’s ground plane by following the four arrows that are parallel to the ground plane. Applying a force in the (−z) direction reduces the height of the helix and increases its radius while applying a force in the (+z) direction reduces the radius of the helix and increases its height. The relationship between the helix height and radius for the length of the strip (l=188.5 mm) and the number of turns (N=0.5 turns) is presented in Fig. 2c.

In the remainder of this section, we identify the particular combinations of radius and height (i.e., helix geometries) that result in the desired radiation pattern and polarization reconfiguration. Accordingly, we then present a material and structural concept to ensure the selected helix geometries correspond to the minima of the deformation strain energy of the structure – making these geometries stable states that require no external energy to be maintained.

Two distinct operational states
The electromagnetic response of the proposed QHA was assessed by simulations for various radius and height combinations as per Fig. 2c. These different geometries were simulated using Ansys High-Frequency Structure Simulator (HFSS)30, discussed in the Methods section. Supplementary Table 1 provides the detailed antenna performance metrics as a function of the helix geometry. It was found that for a radius (R) less than 25 mm, the antenna structure is not well matched. For 25 mm < R < 35 mm, the antenna exhibits a close to omnidirectional pattern. A radius between 45 mm < R < 55 mm allows the antenna to exhibit a directional pattern with circularly polarized radiation behavior. For R > 55 mm, the antenna design is no longer matched.

The simulated and normalized radiation patterns representing these different operating regimes are shown for three different radii in Fig. 2d. It is important to note that for a radius between 35 mm and 45 mm, the antenna starts its operational transition from an omnidirectional pattern to a directional one. Since the conductive strip length remains constant throughout the reconfiguration, the operating frequency remains within the antenna bandwidth between 1 GHz and 1.2 GHz. Accordingly, two distinct antenna operating states are identified. These states reconfigure the radiation characteristics of the antenna. State 1 is characterized by a radius of R=30 mm and an almost omnidirectional radiation pattern. These characteristics make it suitable for terrestrial communications between various devices deployed in a low-infrastructure or disaster-stricken area. On the other hand, State 2 is characterized by a radius of around R=55 mm with a directive radiation pattern and circular polarization, making it ideal for satellite communications.

The corresponding simulated magnitude of the 3D radiated electric field pattern for state 1 and state 2 are presented in Fig. 3a, b at f=1.1 GHz. The 2D left- and right-handed circularly polarized gain patterns along the Y−Z plane are presented for state 2 in Fig. 3c. A cross-polarization of 41.68 dB is obtained along the direction of maximum radiation at θ=0°. This confirms the circular polarization purity of the antenna in state 2.

Multi-stable composite structure
Each helix is composed of initially flat strips of glass fiber and epoxy composites with an embedded conductive mesh. These strips are interconnected using rotational joints into a helical lattice as discussed in the Methods section. As the helix is reconfigured between various geometries, the strips undergo bending and twisting31, thereby changing the strain energy stored in the structure due to the elastic deformation of the material. The stored strain energy, U, results in forces acting along the axis of the helix according to Castigliano’s theorem32,

Fa = \frac{\partial U}{\partial h} (2)

These forces deform the structure towards its minimum energy configuration. The energy minima are the stable points of the structure, where the restoring forces are Fa=0 and the structure can maintain its geometry without external power. In the case that the force pushes the structure towards its minimum or maximum radius, there are additional stable points at these extremes. We leverage such behavior in our study to enable the low-energy, user-centric reconfiguration of the structure. The desired geometries are reliably achieved as axial forces due to the stored strain energy and will always push the structure to the minimum energy configuration. No DC power is required for actuators to maintain stable geometries.

The use of fiber-reinforced polymer (FRP) composites for the strip material has been shown in the literature to yield a rich design space for stability31. The stored strain energy can be predicted as a function of material properties and helix geometry using a 2D analytic model accounting for longitudinal and transverse bending as well as twisting in the strips. A detailed discussion of the design parameters is presented in the Methods section. We demonstrate that we can use the fiber orientation of the composite material relative to the strip length, θ, to induce a multi-stable response. Figure 4a demonstrates the use of fiber orientation in the strips to increase the number of stable points for a helix made of glass-fiber-reinforced epoxy material. It is found that at a fiber angle of 8°, the structure successfully outputs two stable states at R=30 mm and R=55 mm, and electromagnetic reconfiguration. The third stable point at the minimum radius is not used in practice. At this fiber angle, the FRP material has low torsional stiffness as well as high material coupling between bending and twisting deformations. The helical strips twist significantly to enable multi-stability. The stable radii are measured when the QHA is integrated with its ground plane. These agree well with the predictions in Fig. 4a with small deviations due to manufacturing imperfections, the physical width of the strips, and integration with the ground plane. The antenna is shown in its two stable states in Fig. 4b. The resulting dimensions of the antenna structure in the two stable operating states with radii of R=30 mm and R=55 mm are summarized in Fig. 4b. The three stable states (two operational and one non-operational) of the selected design are demonstrated experimentally for a prototype with N=1.5 turns in Supplementary Video 1.

The use of FRP composites for the multi-stable antenna presents a robust implementation for low-infrastructure environments. Multi-stability of FRP composite structures can be maintained over millions of actuation cycles, despite some stiffness reduction of the material under fatigue33. While temperature extremes can shift the location of stable points34, such effects are expected to be minor over the range of temperatures encountered on Earth and can be mitigated through fiber and polymer selection. These small changes in the stable radius have a modest impact on the antenna gain while overall radiation and polarization characteristics remain stable (Supplementary Table 1). Similarly, small vibrations around the stable states will not have a significant impact on electromagnetic performance (Supplementary Table 1). However, dynamic loading (e.g., wind) that causes resonance can inadvertently switch states of the antenna. In fact, dynamic excitation has been proposed as an active actuation strategy for bi-stable structures35. To avoid unintended actuation, the FRP material offers a large design space for tailoring the structural stiffness, and hence resonance frequencies such that they are removed from environmental forcing frequencies. Specifically, we demonstrate that the axial stiffness of the helix can be tuned using the composite fiber angle and strip width while preserving coupling between stable points and antenna reconfiguration (Supplementary Fig. 6). Overall, while state-of-the-art solutions to mechanically reconfigurable antennas trade off mass and portability with environmental robustness (e.g., heavy motor-driven articulated mechanisms36 in contrast to membrane-based origami antennas13), the technology proposed here offers a lightweight, portable structure with robust multi-stability across a large range of environments.

The radio frequency (RF) feeding network
The four conductive strips of the QHA are fed by a dedicated feeding network that is fixed underneath the antenna’s ground plane. The feeding network, shown in Fig. 5, supplies equal amplitude signals with sequential quadrature phases. As shown in Fig. 5a, the feeding network relies on one rat-race coupler and two 3-dB hybrid couplers37 (Supplementary Notes 1, 2).

The rat-race coupler is a reciprocal four-port network that splits the input signal Pin into two equal output signals with a phase shift of 180°. Two of the output ports of the rat-race coupler connect to two 3-dB hybrid couplers as shown in Fig. 5. Each 3-dB hybrid coupler is also a four-port network that divides its input signal into two equal output signals (P1 and P2 or P3 and P4, respectively) with a 90° phase shift as shown in Fig. 5b. All remaining ports are isolated and connected to 50 Ω loads. The combination of a rat-race coupler and the two 3-dB hybrid couplers leads to a quadrature feeding network with equal amplitude signals and sequentially quadrature phases.

Before testing the functionality of the antenna, the feeding network is assessed. The comparison between the simulated and measured S-parameters magnitude of the proposed feeding network is shown in Fig. 5c. Figure 5d presents the simulated and measured phase differences between the various ports of the proposed feeding network. These results prove that the designed network provides equal amplitude signals with sequential quadrature phases.

Ground plane design and sliding feed
The physical realization of the two states of the proposed antenna structure requires sliding the various helical wires along the ground plane as the antenna reconfigures its radius. A customized ground plane is implemented as part of the proposed structure to allow the variation of the antenna’s radius. The typical square ground plane is modified to include four slots coinciding with the motion of the four conductive arms as the antenna changes its radius (Fig. 6a).

Four coaxial cables with SMA connectors are connected to the four conductive helical arms and move along these slots. The ground of each SMA connector is attached via conductive epoxy to a slot cover that slides underneath the respective slot. A fixed distance is maintained between the cover and the ground plane using 3D-printed supports. To ensure the electrical connectivity between the ground plane and the sliding covers, spring-loaded pins with a gold-plated ball bearing at their tips are integrated with the 3D-printed parts. Figure 6b shows a partially assembled support for the sliding covers. The coaxial feeding movement on the ground plane with the state change of the antenna is shown in Fig. 6c.

Photos of the top and bottom views of the ground plane are shown in Fig. 7a, b. Photos of the completed assembly can be seen in Fig. 7c. The manufactured ground plane is fabricated using 0.5 mm thick Copper and has a side length of 374 mm. A comparison between the performance of the QHA over a full ground plane with its performance over a covered slotted ground is shown in Supplementary Note 3. It is demonstrated that the effect of the slots is negligible.

Measured electromagnetic performance of the QHA
The antenna was measured in an anechoic chamber at the Antenna Measurement facility in the EMpact lab at the American University of Beirut. It is a standard ETS Lindgren anechoic chamber38 connected to a Performance Network Analyser (PNA)39, a controller, and a computer to control the turning table and extract the results (Methods). The antenna under test is put in transmitter mode and a horn antenna is used as a receiver.

Figure 7d, e shows the antenna with its own feeding network being tested in an anechoic chamber in its two operating states. The measurement was done in the two stable states of the antenna and the performance was compared to simulation.

The transition of the fabricated antenna from State 1 to State 2 and vice-versa is shown in Fig. 7f, where the fabricated antenna in its two states is presented. Transition from State 1 to State 2 is executed as a result of an applied vertical downward force which, when applied to the tips of the QHA, forces the rotational joints to rotate in the clockwise direction. As a result of this rotation, the conductive strips (brown colored strips in Fig. 7f) rotate in a clockwise and downward direction, the non-conductive strips (white colored strips in Fig. 7f) rotate in a counter-clockwise and downward direction. This increases the radius of the QHA, and transitions it into a stable state 2. On the other hand, a vertical upward force applied to the tips of the QHA in stable state 2, forces the rotational joints to rotate in the counter-clockwise direction. As a result, the conductive strips rotate in a counter-clockwise and upward direction, the non-conductive strips rotate in a clockwise and upward direction, and the radius of the antenna decreases. As a result, the antenna returns to stable state 1.

The comparison between the simulated and measured input reflection coefficients (|S11| [dB]) is presented in Fig. 8a for both states, demonstrating that the antenna preserves the same operational frequency. The great agreement between measured and simulated |S11| demonstrates that the fabricated antenna prototype along its feeding network satisfies the design requirements in terms of frequency operation.

The radiation characteristics of the fabricated prototype in the two deployed states are similarly assessed and compared to simulations. The radiation patterns of the antenna are measured at f=1.1 GHz since this is the frequency of interest for the antenna operation. In state 1, the antenna exhibits an almost omnidirectional radiation pattern as shown in Fig. 8b. As for state 2, the antenna exhibits a directional radiation pattern as depicted in Fig. 8c. For both states, the simulated and measured results show good agreement.

The axial ratio of the second state of the proposed antenna structure can be calculated using (3)40.

AR = \frac{2\lambda S}{(\pi D)^2} (3)

In state 2, the helix spacing is 0.5λ while its radius is 0.1833λ. These dimensions result in a predicted axial ratio of 0.7546 dB, which is less than the 3 dB threshold. Thus, circularly polarized radiation is obtained in state 2. To validate the circular polarization of the antenna in state 2, the simulated and measured axial ratio at f=1.1 GHz are plotted in Fig. 8d over the span of elevation angles where the maximum radiation is focused. It is clear that the axial ratio drops to below 3 dB for a range of angles between ±40° where the maximum radiation is centered, proving that the antenna is circularly polarized.

The antenna exhibits a maximum measured realized gain of 2.6 dB for state 1, while it exhibits a maximum measured realized gain of 8.5 dBic for state 2, which renders state 2 of this antenna suitable for satellite communication. Comparisons between the measured and simulated realized gains for both states of the antenna are shown in Fig. 8e, f. It is clear from Fig. 8 that the measured and simulated results are in excellent agreement, which confirms that the designed antenna achieves its envisioned performance with great fidelity.

Designed QHA Compared to a Conventional QHA
To achieve a normal mode conventional helix, the circumference, C, of the helix, and the spacing, S, between each helical turn, must be less or equal to 1/2 λ. As a result, for a frequency of 1.1 GHz, the radius, R, of the helix must be less or equal to 21.7 mm, and the spacing, S, must be less than or equal to 136.0 mm (Supplementary Table 2). On the other hand, to achieve an axial mode conventional helix40, C must be between 3/4 λ and 4/3 λ in addition to a spacing S of 1/4 λ. As a result, for a frequency of 1.1 GHz, axial mode operation is achieved when the radius and spacing of the helix40 are 32.6 mm < R < 58.0 mm, and S ≈ 68.3 mm. The conventional QHA calculation studies the helical spacing between the values of 0 and λ. However, in our design we went beyond λ in terms of spacing to venture an approach where we achieve a normal mode and an axial mode corresponding to stable geometries from a mechanical perspective. In other words, an unconventional helical design was selected to merge electromagnetic reconfiguration with mechanical stability.

Hence, a major portion of the analysis executed in this paper is focused on identifying the sweet spot between the mechanical stability of the structure and its electromagnetic performance. To ensure deployment of the QHA while maintaining the same frequency of operation, the radius and spacing must be adjusted to maintain the same strip length. So when the radius of the wire increases, its spacing decreases and vice-versa (as in Eq. (1)).

As can be seen from Supplementary Fig. 5, for a conventional QHA at an operating frequency of 1 GHz the total length of the wire l differs between the normal and axial modes. Therefore, the same conventional antenna cannot operate in two different modes within the same frequency range. Our proposed design overcomes this limitation by preserving the same wire length l for all the radii. This was achieved by modifying the spacing between the wires while taking into consideration the spacing-radius relation (Fig. 2c) ensuring that the mechanical stability of the design is preserved.

We can generalize the design space of reconfigurable electromagnetic performance by representing it as a function of the operating wavelength, λ (Fig. 9a). In this way, the discovered reconfiguration principle can be applied to any desired frequency. As can be seen, for a QHA with 0.5 turns, 0.935λ < S < 1.14λ, and 0.0833λ < R < 0.133λ, produces an omnidirectional-like pattern. A QHA with 0.5λ < S < 0.935λ and 0.133λ < R < 0.18833λ is circularly polarized (CP) with a directional pattern. Beyond these limits, the antenna is not resonant (NR). A demonstration of the radiation pattern reconfiguration across a range of operating frequencies is provided in Fig. 9b.

Comparing passive and active actuation
Multi-stability offers a user-centric approach to antenna actuation that eliminates the need for DC power. This means users can manually adjust the antenna to meet their communication requirements, knowing that multi-stability will precisely configure the structure for optimal electromagnetic performance. This operational principle proves especially advantageous in regions with limited infrastructure and power resources.

Alternatively, a multi-stable antenna can also be autonomously reconfigured via powered actuators. It is noteworthy that in this case, multi-stability still offers significant energy savings as the actuation needs to be powered only to reconfigure the structure while multi-stability maintains the antenna geometry in the desired operational states. In addition, the combination of multi-stability and powered actuation can ensure that the antenna maintains its current state in case of power failure and may be advantageous where safety considerations preclude manual reconfiguration. Stimuli-responsive smart materials are an attractive option for the actuation of multi-stable structures35,41,42 instead of traditional electro-mechanical actuators such as motors. Smart material actuators can interface directly with the structure41 or be embedded in the material35,42 to reduce the mass associated with power transmission (e.g., gears). For the antenna proposed here, shape memory alloys present one actuation possibility, if the antenna is proposed for implementation in different circumstances or environments where power is available and not scarce. These materials exhibit phase transitions43 that allow the actuator to hold a large deformation that can be recovered via resistive heating. A pair of shape memory alloy actuators can be used in an antagonistic arrangement for reversible actuation41,44. We present a proof-of-concept antenna deployment using shape memory alloy spring actuators in Supplementary Fig. 7, however, we emphasize the fact that such actuation is only useful in regions where power resources are available contrary to our intended implementation in this work.

On the other hand, the deployment mechanism adopted for this structure eliminates the need to integrate PIN diodes or any other electronic switch into the feeding network or the antenna itself. Our proposed passive bistable deployment technique offers more robustness, and a greater immunity to environmental factors, and eliminates any nonlinearity effects that may result from the integrated switches. Furthermore, a bias-free system implicates less interference in the antenna’s radiation characteristics, especially in a QHA topology which is an out-of-plane topology. Additionally, it is important to emphasize that realizing the unique reconfigurable functionality of our proposed QHA necessitates a mechanical modification of its structural topology, a feature that cannot be achieved using electronic switching reconfiguration.

Methods
Prediction of stability points
An analytic energy-based model that computes the 2-dimensional deformation of the helical strips during the deformation of the lattice is employed to design the FRP composite material that the helical strips are made from29,56. The aim of the model is to design a structure with stable geometries corresponding to the two desired operating states of the antenna. In the most general case, the strain energy of a linear elastic composite material strip due to in-plane and bending deformation is given by:

Ustrip = 1/2 \iint_A \begin{bmatrix} \epsilon^0 \\ \kappa \end{bmatrix}^T \begin{bmatrix} A & B \\ B & D \end{bmatrix} \begin{bmatrix} \epsilon^0 \\ \kappa \end{bmatrix} dudv (4)

where the integral is over the area of the strip and u and v are local coordinates along the length and width of the strip, respectively. A, B, and D are 3×3 matrices representing the FRP composite stiffness relating the force, N, and moment, M, resultants per unit width to the mid-plane strains, ε0, and curvatures, κ57,

\begin{bmatrix} N \\ M \end{bmatrix} = \begin{bmatrix} A & B \\ B & D \end{bmatrix} \begin{bmatrix} \epsilon^0 \\ \kappa \end{bmatrix} (5)

Note that ABD stiffness is distinct for the conductive and dielectric strips. We use a 2D model following work in the literature29,58 for the strip curvature assuming that there is longitudinal and transverse bending as well as twisting in the strips. Only longitudinal strains are considered, with transverse and shear strains assumed to be negligible. From this, we deduce that the largest influence on the stability is the material bending stiffness, D, and the strip width (which has a large effect on the transverse curvature of the strips).

The strip energy in Eq. (4) is summed for all the helical strips in the antenna and the stable points correspond to the minima of the energy. The axial forces exerted during deformation can be computed using Eq. (2).

Selected antenna design
The large mechanical design space is narrowed down using two constraints. First, only symmetric laminates were considered to prevent the helical strips from warping in response to a temperature change during manufacturing. Second, equal strip lengths (defined along the arc length of the helix) for all strips are selected to prevent torsion in the helix. The strip length is fixed at l=188.5 mm to allow operation at approximately 1 GHz. These constraints leave the following variables for the design: the widths of the dielectric and conductive strips (defined transverse to the length of the strip), and the layups of the conductive and dielectric strips (i.e. number of layers in the composite material and the fiber orientation in each layer). The authors previously demonstrated that fiber angles around 10° result in tri-stable lattices with stable states at the minimum and maximum radii as well as an additional state at an intermediate helix configuration56. This effect is shown here for the antenna structure in Fig. 4a. These fiber angles result in the highest ratio of bend-twist coupling vs. torsional stiffness for a unidirectional composite (i.e., D16/D66). Starting from this design, layers with fibers either parallel or perpendicular to the length of the strips were added at the center of the composite to improve transverse strength. The final structural parameters of the antenna design are summarized in Table 1.

Materials and manufacturing. All helical strips are manufactured from E-glass fiber-reinforced epoxy composites. The conductive strips contain a thin woven phosphor-bronze mesh (292 g/m2) at the mid-plane of the laminate. Glass fiber-epoxy prepreg with an areal weight of 25 g/m2 is used and is laminated to the desired layup, and it is cured for 2 hours at 120 °C in an autoclave. The material properties of a single glass fiber-epoxy layer are summarized in Supplementary Table 3. The strips are assembled into a helix using nylon screws. To reduce joint friction, Teflon washers are used between the strips. Friction in the joints can affect the stable points and reduce repeatability.

Simulation of the antenna performance. Each of the two stable states of the antenna was simulated using Ansys HFSS30 to evaluate the electromagnetic performance of the antenna structure. The non-conductive strips are modeled as a dielectric material with a relative permittivity of 3.7 and a loss tangent of 0.001. The number of turns of each helical wire is equal to 0.5 while its ground plane is 374×374 mm2.

Measurement of the radiation pattern, polarization and gain. Testing the antenna is carried out in a standard anechoic chamber from ETS Lindgren38 at the American University of Beirut. The Anechoic cells operating in the radio frequency (RF) waveband must be insulated and isolated from electromagnetic noise coming from outside the test field59. The anechoic chamber contains a space that is lined with radio wave-absorbing material. These absorbers are usually in the form of pyramid-shaped wedges that cover the inside surfaces of the chamber. The test cell structure is designed to resemble a Faraday cage, with a metal sheet serving as the external skin. The absorbers absorb the reflections of the reference signal. At one end of the chamber a horn radiator is placed operating at microwave frequencies. The antenna under test (AUT) is mounted on a rotational table at the other end of the chamber. Elevation and azimuth plots can be plotted by positioning the antenna in the correct plane for each test38. The chamber used for testing the antenna is a fully anechoic chamber, where all surfaces comprising the floor are non-reflective (energy absorbent).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors have proposed and studied a manually deployable Quadrifilar Helix Antenna (QHA) with\nradiation pattern/polarisation reconfigurability. The performance of two states of QHA was simulated\nand tested in experiment.\nThe proposed reconfigurable mechanism of the QHA seems interesting. However, the basic idea of this\nwork was already presented in a major conference in this area [28]. For the application mentioned by\nthe authors, a manually deployable QHA is not convenient. A few reconfigurable QHAs through\nelectronical means were reported recently [a, b]. I would suggest the authors to add a mechanical\nactuation set-up to reconfigure the QHA through an electrical means and re-submit their work to an\nIEEE journal.\na. X. Yi, et al, ‘Polarization and Pattern Reconfigurable Cuboid Quadrifilar Helical Antenna’,\nIEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 66, NO. 6, JUNE 2018.\nb. W. Sun, et al, ‘A Pattern Reconfigurable Circularly Polarized Quadrifilar Helix Antenna Through\nPhase Control’, IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 70, NO. 9, SEPTEMBER\n2022.", "reviewer_2": "The authors present a novel mechanically and electromagnetic reconfigurable quadrifilar helix\nantenna. The use of FRP composites allowed a self-stable locking geometry that corresponded to a\ndifferent electromagnetic operating regimes.\nThe supplementary video demonstrated the disconnected antenna's ability to maintain its differing\noperational regimes based on minor inputs and switched fully with a large enough force applied.\nThere is a need for fieldable ruggedized antennas for harsh and remote environments, and the main\nstructure of the antenna fits that need, but there are a few questions that would need to be answered,\nsuch as:\n1. what is the expected lifetime or has lifetime analysis of the operational locking geometry being\nperformed,\n2. what are temperature extremes effect on both lifetime of the joints and self-stability of the joints\nperformance (such as, does being in the dessert or artic cause the joints to bind),\n3. vibration analysis of structure and its effect on electromagnetic performance,\n4. if this structure is placed outdoors in high winds could some mechanical resonance be achieved that\ncauses it to flip between the bi-stable states (self-resonance of a structure causing failure- similar to\nthe Tacoma Narrows bridge collapse),\n5. how does this technology compares in performance (lifetime, antenna applications, environment\nranges, etc.) to other mechanical reconfigurable antenna technologies,\n6. and lastly, does this technology have any other applications or examples of being applied to other\nantenna types?", "reviewer_3": "This article proposes a Deployable Quantum-Hall Array (QHA) that is capable of passively\nreconfiguring its polarisation state and radiated beam direction. The study exhibits a notable level of\ninterest; yet, there are several comments that necessitate attention from the authors.\n1) Why did the author choose for passive reconfiguration instead? In today's world, a significant\nproportion of systems and processes operate in an autonomous or active way.\n2) Line 91- Is it correct the number of turns of the helix is only 0.5?\n3) Line 259 - \"the spacing is greater than lambda to venture an approach where we achieve a normal\nmode and an axial mode corresponding to stable geometries from a mechanical perspective\". What is\nthe rationale for the necessity of increasing the spacing, considering that the established standard\nrange has already demonstrated its capability to attain both normal mode and axial mode? If the\nspacing and radius are increased, it is expected that the size of the antenna will increase as well.\n4) Figure 2: In practical scenarios, how can the transition from State 1 to State 2 be implemented?\n5) Line 146 and Line 299 - Is there any empirical evidence or analysis available to support the claim\nthat the proposed design exhibits low energy consumption?\n6) Line 101 - 104 - Are there any figures available to support or substantiate those statements?"}, "author_response": "We sincerely thank the reviewers for their constructive feedback, which helped us improve the\ntechnical quality of the paper. We have carefully revised the paper to address the reviewers’\ncomments. In this document, the comments are presented in blue followed by our point-by-point\nresponses.\n\nReviewer 1 Comments:\n\nR1C1: The authors have proposed and studied a manually deployable Quadrifilar Helix Antenna\n(QHA) with radiation pattern/polarization reconfigurability. The performance of two states of\nQHA was simulated and tested in experiment. The proposed reconfigurable mechanism of the\nQHA seems interesting. However, the basic idea of this work was already presented in a major\nconference in this area [28].\nResponse to R1C1: We thank the reviewer for the comment, which allows us to compare the work\npresented in this manuscript with the work presented in [28]. In fact, [28] is a brief conference\npaper of two pages in length. It presents the basic idea of the proposed deployable antenna with\nonly simulated results. On the other hand, in this manuscript, we present the entirety of the work\nin both simulation and measurement and from both structural and electromagnetic perspectives. In\nthis work, we present the fully implemented and integrated feeding network and reconfigurable\nground plane as well as the testing of the entire system in all its states of operation. The manuscript\nis also enriched with a deep dive analysis into the electromagnetic and mechanical performance of\nthe proposed design with plenty of details on the fabricated prototype. Moreover, we present in\nthis manuscript a comparison between the conventional QHA and our proposed structure (Lines\n283-316). Furthermore, a generalized equation for the design of a reconfigurable QHA at any\nfrequency was formulated with the prediction methods of stability points discussed.\n\nR1C2: For the application mentioned by the authors, a manually deployable QHA is not\nconvenient. A few reconfigurable QHAs through electronical means were reported recently [a, b].\nI would suggest the authors to add a mechanical actuation set-up to reconfigure the QHA through\nan electrical means and re-submit their work to an IEEE journal\na. X. Yi, et al, ‘Polarization and Pattern Reconfigurable Cuboid Quadrifilar Helical Antenna’,\nIEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 66, NO. 6, JUNE 2018.\nb. W. Sun, et al, ‘A Pattern Reconfigurable Circularly Polarized Quadrifilar Helix Antenna\nThrough Phase Control’, IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL.\n70, NO. 9, SEPTEMBER 2022\nResponse to R1C2: We agree with the reviewer that electrically reconfigurable QHAs are\nproposed in the literature such as the ones proposed by the reviewer [a,b]. However, the fact that\nthese antennas require electrical reconfiguration necessitates additional power requirements to be\nimposed on the structure. A constraint that we are trying to minimize in our design. Hence, we\nhave intentionally avoided the use of active components such as switches or actuation mechanisms,\nwhich would require external biasing and hence, additional power. The objective of our proposed\nwork is to restore communication in disaster-stricken regions where a shortage/lack of power\npresents a major issue, hence we resorted to designing a manually deployable antenna. The\nproposed deployable antenna does not need any additional power (dc power, etc.) to reconfigure\nits electromagnetic characteristics. The manual deployable and passive reconfiguration is possible\ndue to the bi-stability characteristic of the structure, which not only enables state switching but\nalso maintains the stability of the structure once a state is reached.\nWe have rewritten parts of the section “Discussion” (lines 370-380) in the revised manuscript to\naddress this issue as well as we added references (a) and (b) as references (53), and (54) in our\nrevised manuscript.\nOn the other hand, we do agree with the reviewer that autonomous mechanical actuation may be\nadvantageous in other environments for convenience. As such, we propose the use of shape-\nmemory alloy actuators for reconfiguration in such scenarios. These are metallic alloys that\ndemonstrate a temperature-dependent transformation in their crystal structure. They can be\nstretched to retain strains of ~5% axially that can be recovered upon heating due to the shape\nmemory effect. Two shape memory actuators can be used antagonistically to achieve reversible\nactuation. While the demonstration of reversible actuation is beyond the scope of the work, we\nadded a proof-of-concept of antenna deployment using shape memory alloy springs in\nSupplementary Figure 7 and it is shown below.\nSupplementary Figure 7: Deployment demonstration using shape memory alloy springs. Shape memory\nalloy springs are stretched to the desired actuation stroke and resistive heating is used to heat up springs\nallowing them to recover the stroke (shape memory effect) and deploy the antenna. The springs are made\nof NiTi with an austenite finish temperature of 45C, a wire diameter of 1 mm, and a spring diameter of\n4.75 mm. A fixed actuation current of 5A is used. The unstretched and stretched lengths of the actuators\ncorrespond to stable states 1 and 2, respectively.\nWe added a section ‘Comparing Passive and Active Actuation’ to the main text to summarize this\ndiscussion (Lines 318-336) as well as supporting references (35), (42-45).\nWe would like to thank Reviewer 1 again for taking the time to review the paper and provide\nfeedback that helped us further improve the content of the paper and we hope that we have\nanswered all the concerns that the reviewer presents.\n\nReviewer 2 Comments:\n\nThe authors present a novel mechanically and electromagnetic reconfigurable quadrifilar helix\nantenna. The use of FRP composites allowed a self-stable locking geometry that corresponded to\na different electromagnetic operating regime. The supplementary video demonstrated the\ndisconnected antenna's ability to maintain its differing operational regimes based on minor inputs\nand switched fully with a large enough force applied. There is a need for fieldable ruggedized\nantennas for harsh and remote environments, and the main structure of the antenna fits that need,\nbut there are a few questions that would need to be answered, such as:\nR2C1. what is the expected lifetime or has lifetime analysis of the operational locking geometry\nbeing performed,\nResponse to R2C1: Research has demonstrated that bi-stable structures made of fiber-reinforced\npolymer (FRP) composites continue to exhibit bi-stability after millions of fatigue cycles, despite\na small reduction in material stiffness due to fatigue. We added a comment regarding these results\nin lines 165-167 and have added reference (33) to the revised manuscript.\nR2C2. what are temperature extremes effect on both lifetime of the joints and self-stability of the\njoints performance (such as, does being in the dessert or artic cause the joints to bind),\nResponse to R2C2: The stability of composite structures can be affected by temperature extremes,\nas happened on the composite radar instrument on the Mars Express spacecraft orbiting Mars when\nan attempted deployment of the radar at -70C showed an additional stable state not observed at\nroom temperature. In this work, we anticipate operation in more moderate temperature ranges\nencountered on Earth. The stable states are expected to shift only slightly due to the temperature-\ndependent moduli of the composite material constituents. This can also be minimized through\ncareful material selection. Small shifts in the stable states are expected to have a small impact on\nthe gain while the radiation pattern characteristics remain stable (this is demonstrated in\nSupplementary Table 1). Regarding the joints, the most important design consideration is to reduce\nfriction in the joints. In this work, we use Teflon spacers between the strips to achieve this. A more\nrobust implementation can use sealed bearings and an appropriate lubricant to provide reliable\noperation in dusty environments across a broad range of temperatures. These components are\navailable off-the-shelf.\nWe summarize this discussion by adding lines 167-171 to the main text of the revised manuscript\nas well as reference (34).\nR2C3. vibration analysis of structure and its effect on electromagnetic performance,\nResponse to R2C3: Similar to how small changes in the stable radius do not impact the overall\nradiation pattern significantly, small vibrations around the stable states are also expected to\npreserve the radiation pattern with a small impact on the gain (Supplementary Table 1). We have\nadded lines 171-172 to the main text of the revised manuscript to summarize this.\nR2C4. if this structure is placed outdoors in high winds could some mechanical resonance be\nachieved that causes it to flip between the bi-stable states (self-resonance of a structure causing\nfailure- similar to the Tacoma Narrows bridge collapse),\nResponse to R2C4: Larger vibrations can cause resonance and switching between stable states.\nThis can be used as an active actuation strategy or avoided by shifting the structure’s resonance\nfrequencies in both stable states away from environmental forcing frequencies (e.g., wind). We\nadded an analysis to demonstrate that the axial stiffness of the structure in the two stable states\n(and hence the resonance frequencies) can be tuned without impacting the location of the stable\nstates. We have also added Supplementary Figure 6 to summarize this analysis.\nSupplementary Figure 6. Tailoring axial stiffness of two stable states, 𝐾 and 𝐾 , using the fiber angle, 𝜃,\n1 2\nand dielectric strip width, 𝑤. The following are used for the analysis: fixed conductive strip width of 8 mm,\nconductive strip layup of [𝜃𝐺𝐹𝑅𝑃/0𝑃𝐵/𝜃𝐺𝐹𝑅𝑃], and dielectrip strip layup of [−𝜃𝐺𝐹𝑅𝑃/90𝐺𝐹𝑅𝑃] . (a) Stable\n7 𝑝𝑤 7 4 3 𝑠\nradius in state 1. Note that the stable radius of state 2 is always at the maximum possible radius. (b)\nResulting axial stiffness in two stable states. The axial stiffness can be tailored to control the structural\nresonance in the two states under the constraint of a desired stable radius (e.g., R = 30 mm as in the red line\nin (a)).\nIn addition, we added a few lines summarizing this discussion and new analysis to the main text\nin lines 173-179.\nR2C5. how does this technology compares in performance (lifetime, antenna applications,\nenvironment ranges, etc.) to other mechanical reconfigurable antenna technologies,\nResponse to R2C5: The greatest advantage of this technology over other mechanically\nreconfigurable antennas is that we achieve large electromagnetic reconfiguration while\nsimultaneously achieving stable operating states, thereby reducing the energy consumption of the\nsystem. This is achieved by coupling the mechanical and electromagnetic design. Advantages over\nother bi-stable designs are discussed in detail in lines 350-363.\nAdditionally, the proposed technology presents a lightweight design leveraging compliant\nmechanisms for reconfiguration while the FRP composite material provides robust operation in a\nrange of environments. This is in contrast to existing concepts that are either portable or\nenvironmentally robust. We added few lines to the main text in lines 179-182 to summarize this\ndiscussion.\nR2C6. and lastly, does this technology have any other applications or examples of being applied\nto other antenna types?\nResponse to R2C6: The use of bistable material has been discussed in the literature for\napplications in deployable antennas or deployable booms for various space structures. However,\nwhile previous work has considered the use of mechanical bi-stability for antennas, bi-stability\nwas decoupled from the antenna operation. Deployable antennas where the stowed and deployed\nstates are both stable are presented in the literature as in references (46) and (47) in the revised\nmanuscript. In this case, only the deployed state operates as an antenna. Other studies have\ninvestigated the effects of bi-stable structures on electromagnetic performance as discussed in\nreferences (48) and (49) of the revised manuscript. While some minor reconfiguration of operating\nfrequency or beam direction was demonstrated, the antenna performance was significantly\nconstrained as the bi-stable geometries could not be modified. For example, bistable composite\nlaminates were employed in the design of a microstrip patch antenna as presented in reference\n(50), or as part of composite tape springs for deployable monopoles out of CubeSat shroud as in\nreference (46). However, our presented design can be considered more advantageous than the\navailable work in the literature as we demonstrated a truly coupled performance between\nmechanical stability and electromagnetic performance, without resorting to any form of external\nactuation. On the other hand, existing passive and deployable reconfiguration of multifunctional\nantennas presented in the literature as in references (51) and (52) do not exhibit the same capability\nin antenna performance as they focus on a single parameter reconfiguration typically the frequency\nof operation without additional controllable reconfigurable performance metrics.\nThe third paragraph of the discussion section was modified to reflect this discussion (lines 350-\n363).\nWe would like to thank Reviewer 2 again for taking the time to review the paper and provide\nfeedback that helped us further improve the content of the paper and we hope that we have\nanswered all the concerns that the reviewer presents.\n\nReviewer 3 Comments:\n\nR3C1: This article proposes a Deployable Quantum-Hall Array (QHA) that is capable of passively\nreconfiguring its polarisation state and radiated beam direction. The study exhibits a notable level\nof interest; yet, there are several comments that necessitate attention from the authors. Why did\nthe author choose for passive reconfiguration instead? In today's world, a significant proportion of\nsystems and processes operate in an autonomous or active way.\nResponse to R3C1: The antenna is purposefully designed to operate in disaster-stricken regions\nwhere power is scarce if not lacking. As a result, passive reconfiguration achieves this goal by\nproviding a dynamic antenna operation without spending additional non-existing power on the\nactuation mechanism. It enables the antenna to achieve dual functionality without requiring\nadditional power. The tradeoff between passive and active actuation is discussed in a newly added\nsection in lines 318-336.\nR3C2: Line 91- Is it correct the number of turns of the helix is only 0.5?\nResponse to R3C2: Yes, indeed, the number of turns of the helix antenna is only 0.5. The\nprovided supplementary video demonstrates the bi-stability and deployment of the structure on a\nlonger design.\nR3C3: Line 259 - \"the spacing is greater than lambda to venture an approach where we achieve a\nnormal mode and an axial mode corresponding to stable geometries from a mechanical\nperspective\". What is the rationale for the necessity of increasing the spacing, considering that the\nestablished standard range has already demonstrated its capability to attain both normal mode and\naxial mode? If the spacing and radius are increased, it is expected that the size of the antenna will\nincrease as well.\nResponse to R3C3: A major portion of the analysis executed in this paper is focused on\nidentifying the sweet spot between the mechanical stability of the structure and its electromagnetic\nperformance. To be able to achieve deployment of the QHA and conserve the same frequency of\noperation, the radius and spacing should change to maintain the same total length of the wire l,\nwhere the number of turns is N=0.5. So, when the radius of the wire increases, its spacing\ndecreases and vice-versa.\nAs can be seen from the newly added Supplementary Figure 5, shown below, for normal and axial\nmodes of a conventional QHA at the frequency of 1 GHz the total length of the wires differs. So,\nthe same conventional antenna cannot operate in two different modes in the same frequency range.\nOur proposed design was able to overcome this by preserving the same wire length for all the\nradii. This was achieved by modifying the spacing between the wires while taking into\nconsideration the spacing-radius relation ensuring that the mechanical stability of the design is\npreserved as shown in Fig. 2(c) of the manuscript.\nThe figure below was added to the supplementary document as supplementary Figure 5.\nSupplementary Figure 5: Total length of each wire of the conventional QHA at the frequency of 1 GHz for\naxial mode and normal mode in addition to our proposed structure\nR3C4: Figure 2: In practical scenarios, how can the transition from State 1 to State 2 be\nimplemented?\nResponse to R3C4: In practical scenarios, the transition from State 1 to State 2 and vice-versa is\nshown in Figure 7(f) in the revised manuscript (also shown below), where the fabricated antenna\nin its two states is presented. As can be seen, when a vertical downward force is applied to the tips\nof the QHA in stable state 1, the conductive strips (brown colored strips) will rotate clockwise\naround the joints, the non-conductive strips (white colored strips) will rotate counterclockwise\naround the joints, and the radius of the QHA antenna will increase while the height decreases. So,\nthe antenna will transition to a stable state 2. On the other hand, as can be seen in the same figure,\nwhen a vertical upward force is applied to the tips of the QHA in stable state 2, the conductive\nstrips will rotate counterclockwise around the joints, the non-conductive strips will rotate\nclockwise around the joints, and the radius of the antenna will decrease while the height increases.\nSo, the antenna will return to stable state 1.\nFigure 7 (f) in revised manuscript: State variation directions for the fabricated prototype\nR3C5: Line 146 and Line 299 - Is there any empirical evidence or analysis available to support\nthe claim that the proposed design exhibits low energy consumption?\nResponse to R3C5: Since the antenna is manually deployable and reconfigurable, it does not need\nadditional electrical power for deployment including the use of actuators or external biasing to\nswitch between different states. The only power needed is the power for the input of the\ntransceiver, which is envisioned to be connected to the antenna as a back-end circuitry to ensure\ncommunication. A potential transceiver is an off-the-shelf component from Analog Devices,\ndiscussed in (55) which requires 1.3 V and 2.9 mA for operation.\nR3C6: Line 101 - 104 - Are there any figures available to support or substantiate those statements?\nResponse to R3C6: We updated Supplementary Table 1 (also shown below) to support these\nstatements. In fact, it was found that for a radius (R) less than 25 mm, the antenna structure is not\nwell matched, and the maximum realized gain is negative. For 25 mm < R < 35 mm, the antenna\nexhibits a close to omnidirectional pattern. A radius between 45 mm < R < 55 mm allows the\nantenna to exhibit a directional pattern with circularly polarized radiation behavior. For R > 55\nmm, the antenna design is no longer matched.\nSupplementary Table 1: Antenna performance\nRadius Radius S-parameter Radiation pattern AR Frequency\npec dielectric (GHz)\n(mm) (mm)\n60 60.6 CP 1.143 No matching\n55 55.6 CP 1.115 Matching/\nDirectional\nradiation\npattern\n40 40.6 CP 1.094 Matching/\nTransition\nstate\n35 35.6 NCP 1.087 Matching/\nTransition\nstate\n30 30.6 NCP 1.08 Matching/\nOmni-\ndirectional\nradiation\npattern\n25 25.6 NCP 1.073 Matching/\nOmni-\ndirectional\nradiation\npattern\n20 20.6 NCP 1.0485 No matching/\nNegative gain\n10 10.6 NCP 0.9505 No matching/\nNegative gain\nWe would like to thank Reviewer 3 again for taking the time to review the paper and provide\nfeedback that helped us further improve the content of the paper and we hope that we have\nanswered all the concerns that the reviewer presents."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Basically, I am not satisfied with the responses from the authors to my comments below.\n‘The proposed reconfigurable mechanism of the QHA seems interesting. However, the basic idea of\nthis work was already presented in a major conference in this area [28].’\nThe authors have admitted that the basic concept, i.e. the principle of the work was already\npresented in their conference paper. The experimental verification and the conventional feeding\nnetwork being implemented do not generate much novelty.\n‘For the application mentioned by the authors, a manually deployable QHA is not convenient. A few\nreconfigurable QHAs through electronical means were reported recently [a, b]. I would suggest the\nauthors to add a mechanical actuation set-up to reconfigure the QHA through an electrical means and\nre-submit their work to an IEEE journal.\na. X. Yi, et al, ‘Polarization and Pattern Reconfigurable Cuboid Quadrifilar Helical Antenna’, IEEE\nTRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 66, NO. 6, JUNE 2018.\nb. W. Sun, et al, ‘A Pattern Reconfigurable Circularly Polarized Quadrifilar Helix Antenna Through\nPhase Control’, IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 70, NO. 9, SEPTEMBER\n2022.’\nThe authors’ suggestion on the additional electrical power consumption to operate the PIN diode or RF\nswitches doesn’t stand. The power for driving these electronic switches is in the order of a few - tens\nmW. For example, the PIN diode (Infineon BAR50-02L) in Ref [b] only needs about 50mW to operate.\nAlso, the electronic switching time is very short, in the order of hundreds of nanosecond, and\nconsumes very little energy. This energy burden is nominal in the application suggested by the\nauthors, in which the transceiver needs a power supply in the order of tens - hundreds W. Of course,\nthe electro-mechanical means proposed by the authors consume more energy and may add an\nadditional burden to the power supply.\nWith the advancement of electronic technology, an electronic means for re-configuring an antenna\nis far superior than the mechanical or electro-mechanical ones. The proposed work may have some\nmerits 30 or 40 years ago. It was the reason why I have suggested that the work is better to be\nevaluated in the IEEE antenna community.", "reviewer_2": "The revised version of the manuscript has highlighted clearly the contribution of this work to the field.\nI do not have any other comments.", "reviewer_3": "The authors have addressed the comments appropriately, but I have a concern about the following\nstatement.\n\"To address this challenge, the reconfiguration mechanism in this work is made completely passive by\nrelying on a manual reconfiguration process, thus removing the need for dc power and external\nbiasing\"\nThe author intended to implement this antenna design in a disaster-prone area; does it make sense\nfor them to risk their lives by reconfiguring the antenna manually? At least you have a backup\npower source to keep the device operational."}, "author_response": "We sincerely thank the reviewers for their constructive feedback, which helped us improve the\ntechnical quality of the paper. We have carefully revised the paper to address the reviewers’\ncomments. In this document, the comments are presented in blue followed by our point-by-point\nresponses. All changes to the manuscript are colored in red in the revised manuscript document.\n\nReviewer 1 Comments:\n\nR1C1: Basically, I am not satisfied with the responses from the authors to my comments below.\n‘The proposed reconfigurable mechanism of the QHA seems interesting. However, the basic idea\nof this work was already presented in a major conference in this area [28].’\nThe authors have admitted that the basic concept, i.e. the principle of the work was already\npresented in their conference paper. The experimental verification and the conventional feeding\nnetwork being implemented do not generate much novelty.\nResponse to R1C1: We thank the reviewer for the comment. It is important to note that\nconference presentations serve as a platform to discuss novel ideas and concepts in a brief\nsummarized manner to spark discussions, which is a clear, well-established norm in the scientific\ncommunity. This practice does not infringe upon the comprehensive publication of a journal paper\nwhere intricate details of the design process, its novelty, contribution, and distinction are presented.\nIn addition, in this manuscript, we have not only acknowledged this principle but also taken\nadditional measures to underscore the significant contributions of our work beyond what is briefly\ndiscussed in the conference presentation. This was discussed in detail in our previous response and\nis summarized herein:\nThe novelty of the work resides in the merging of structural and electromagnetic perspectives from\nthe initial design stage to the full implementation of the antenna system, its reconfigurable\ndeployment mechanism, and its bi-stability along its fully dedicated feeding network all compose\nthe novel aspect of this work. In addition, the comparison between any conventional QHA with\nour proposed topology and the design generalization with stability points prediction opens doors\nfor future researchers to build upon this work to innovate further.\n\nR1C2: ‘For the application mentioned by the authors, a manually deployable QHA is not\nconvenient. A few reconfigurable QHAs through electronical means were reported recently [a, b].\nI would suggest the authors to add a mechanical actuation set-up to reconfigure the QHA through\nan electrical means and re-submit their work to an IEEE journal.\na. X. Yi, et al, ‘Polarization and Pattern Reconfigurable Cuboid Quadrifilar Helical Antenna’,\nIEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 66, NO. 6, JUNE 2018.\nb. W. Sun, et al, ‘A Pattern Reconfigurable Circularly Polarized Quadrifilar Helix Antenna\nThrough Phase Control’, IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION,\nVOL. 70, NO. 9, SEPTEMBER 2022.’\nThe authors’ suggestion on the additional electrical power consumption to operate the PIN diode\nor RF switches doesn’t stand. The power for driving these electronic switches is in the order of a\nfew - tens mW. For example, the PIN diode (Infineon BAR50-02L) in Ref [b] only needs about\n50mW to operate. Also, the electronic switching time is very short, in the order of hundreds of\nnanosecond, and consumes very little energy. This energy burden is nominal in the application\nsuggested by the authors, in which the transceiver needs a power supply in the order of tens -\nhundreds W. Of course, the electro-mechanical means proposed by the authors consume more\nenergy and may add an additional burden to the power supply.\nWith the advancement of electronic technology, an electronic means for re-configuring an antenna\nis far superior than the mechanical or electro-mechanical ones. The proposed work may have some\nmerits 30 or 40 years ago. It was the reason why I have suggested that the work is better to be\nevaluated in the IEEE antenna community.\nResponse to R1C2: We agree with the reviewer that electrically reconfigurable antennas using\nswitches constitute a powerful choice. However, this choice is not suitable for the application and\nthe design perspective that we are tackling in this work. In fact, in this work, we rely on the bi-\nstability of the material composing the structure to ensure reconfiguration, a principle that\nguarantees passive reconfiguration mechanisms.\nIn addition, it is important to note that we are not proposing the use of any actuators in any way.\nWe specifically indicated that the introduction of mechanical actuators falls beyond the scope of\nthe work, which fully relies on the structure’s bi-stability feature. We introduced the shape memory\nalloy actuator in our previous response to reply to the comment raised by the reviewer and to\nindicate that such an option is not valid in our scenario, even if it can automate the deployment\nprocess in its two states.\nOn the other hand, the deployment mechanism adopted for this structure eliminates the need to\nintegrate PIN diodes or any other electronic switch into the feeding network or the antenna itself.\nOur proposed passive bistable deployment technique offers more robustness, and more immunity\nto environmental factors, and eliminates any nonlinearity effects that may result from the\nintegrated switches. Furthermore, a bias-free system implicates less interference in the antenna’s\nradiation characteristics, especially in a QHA topology which is an out-of-plane topology.\nAdditionally, it is important to emphasize that realizing the unique reconfigurable functionality of\nour proposed QHA necessitates a mechanical modification of its structural topology, a feature that\ncannot be achieved using electronic switching reconfiguration.\nWe added a paragraph between Lines 339 and 347 to clarify this matter.\nWe would like to thank Reviewer 1 again for taking the time to review the paper and provide\nfeedback that helped us further improve the content of the paper and we hope that we have\nanswered all the concerns that the reviewer presents.\n\nReviewer 2 Comments:\n\nThe revised version of the manuscript has highlighted clearly the contribution of this work to the\nfield. I do not have any other comments.\nWe would like to thank Reviewer 2 again for taking the time to review the paper and provide\nfeedback that helped us further improve the content of the paper and we hope that we have\nanswered all the concerns that the reviewer presents.\n\nReviewer 3 Comments:\n\nR3C1: The authors have addressed the comments appropriately, but I have a concern about the\nfollowing statement. \"To address this challenge, the reconfiguration mechanism in this work is\nmade completely passive by relying on a manual reconfiguration process, thus removing the need\nfor dc power and external biasing\"\nThe author intended to implement this antenna design in a disaster-prone area; does it make sense\nfor them to risk their lives by reconfiguring the antenna manually? At least you have a backup\npower source to keep the device operational.\nResponse to R3C1:\nWe thank the reviewer for the detailed examination of the proposed operating scenarios. We agree\nthat safety concerns can arise in certain disaster-stuck areas that may affect the ability of the user\nto reconfigure the antenna. However, our overall intention is to deploy this antenna when needed\nfor search and rescue in a disastrous situation. On the other hand, we note that in general low-\ninfrastructure areas, bi-stability offers a low-power reconfiguration option for agile\ncommunications. If actuation is a must, then one interesting possibility is to have powered\nactuators as a backup, as noted by the reviewer. In this case, bi-stability is advantageous since the\nactuators need not be powered continuously and the shape will be maintained in the case of power\nloss to the actuators. We add this consideration to our existing discussion about a powered vs.\npassive system in lines 326-328.\nWe would like to thank Reviewer 3 again for taking the time to review the paper and provide\nfeedback that helped us further improve the content of the paper and we hope that we have\nanswered all the concerns that the reviewer presents."}]

Decision letter