Keywords

1 Introduction

In the last decade, reconfigurable antennas have drawn significant attention from researchers due to the increase in demand for their functionality in modern communications. Reconfigurable antennas have the capability of changing or switching various characteristics i.e., frequency, radiation pattern and polarization. For that reason, they have been reported to be used in various applications such as communication systems, military devices, cognitive radio systems, and biomedical applications [1,2,3].

Reconfigurability techniques in radio components can be implemented by using either electrical, optical, mechanical, or material-based methods. Microelectromechanical switch (MEMs), PIN diode and varactor diode can be categorized as electrical methods. They generally offer ease of fabrication and cost efficiency, at the expense of more complex designs due to the need for biasing lines and circuits. On the other hand, photoconductive switching is typically integrated with optical or laser beams, which overcomes the need for biasing lines. However, this technique requires sufficiently powered and accurate beam generation to actuate photoconductive diodes in antennas and other radio components. Smart materials offer a low profile and potentially light-weight solution. Depending on the type of materials, this solution is generally applicable as antenna substrates and therefore, are reconfigured on a larger scale with lower resolutions. Besides that, reconfiguration of these materials requires active devices to actuate changes in voltages, temperature, etc. On the other hand, mechanical reconfiguration techniques for antennas such as beam steering using motors also require active devices and may face wear-and-tear issues. Therefore, there is a need for a suitable and sustainable method without the need of integrating any active devices, and a good potential method for reconfiguring antenna properties is by changing its physical structure [1,2,3].

In general, main methods in actuating physical changes in antennas include using microfluidics, origami-based techniques, and rotating structures. A fascinating and potentially passive reconfiguration method is by using the gravitational force [4,5,6]. For instance, researchers in [4] characterized their own substance to build two non-miscible dielectric liquid layers to enable beam steering in a fixed upwards direction for a vehicle-mounted satellite communication system. On the other hand, liquid metal is used in [5] as the patch in rectangular patch antenna with frequency-reconfigurability to operate in 5G mobile network based on gravity and movement of the antenna.

In this research, a planar antenna which can be reconfigured passively using gravitational force is designed to operate in 5.8 GHz industrial, scientific, and medical (ISM) band using liquid metal. The Eutectic gallium-indium (EGaIn) will be selected as liquid metal due to low melting point and low toxicity [5]. Its main beam is directed towards a specific direction following the reflection of the liquid metal-filled cavity that surrounds the patch. Besides that, an additional set of directors strategically located at different adjacent positions to the patch is also considered in this investigation. Besides applying passive gravitational force for reconfiguration, this antenna is innovative from previous research as it offers higher directivity due to the use of directors to improve the directional pattern. The following section will present the design and operation of the proposed antenna, whereas the simulated behavior and performance of the antenna is discussed in Sect. 3. Our concluding remarks are presented in Sect. 4.

2 Antenna Design

2.1 Antenna Structure

Figure 1 depicts the structure of the proposed antenna. It is designed with a simple rectangular patch antenna with partial ground to ease radiation control. A coaxial feed method is used to minimize the effects of feeding on the reconfiguration of radiation patterns towards different directions. This concept is inspired by the concept of Yagi-Uda radiator-director coupling and is made more innovative using liquid metal. In this work, the liquid metal is EGaIn, which features a conductivity of σ = 3.46 x 106 S/m, density = 6280 kg/m3 and viscosity = 0.002 Pa•s A volume of 160.34 mm3 is prefilled into a cavity that surrounds copper patch antenna, which is created on a 1 mm-thickness PDMS substrate (εr = 2.75) and located on a 1.6 mm-thick FR4 substrate (εr = 4.3, tanδ = 0.025). Note that the patch and EGaIn-filled PDMS are completely isolated from each other. When the antenna rotates, the liquid will flow to the lowest point of this cavity due to gravity and act as a wave reflector.

Fig. 1.
figure 1

The structure of the proposed reconfigurable antenna.

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2.2 Directors

In this subsection, directors will be added adjacent to the structure to study the behavior of the main radiation patterns and the improvement in gain. Three main cases are investigated: without directors, with corner directors (90° apart) and with side and corner directors (45° apart), as illustrated in Fig. 2. In addition to that, antenna configurations with larger substrates (70 × 70 mm2 instead of the 60 × 60 mm2) and longer directors (10 mm instead of the proposed 6 mm) will also be studied.

Fig. 2.
figure 2

The position of the directors of the proposed reconfiguration antenna; (a) without directors, (b) with corner directors (90° apart), (c) with side and corner directors (45° apart).

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Throughout this work, the antenna is assumed to be operated when placed in a vertical direction (xy plane), as it would be in practice when mounted on a wall or attached to vehicle. This enables the liquid metal inside the cavity of the antenna to be reconfigured when tilted at eight angles, each at 0°, 45°, 90°, 135°, 180°, 225°, 270° and 325°. The performance of reconfiguration in terms of reflection coefficient (S11), gain, bandwidth and radiation patterns will then be observed and analyzed. All simulations are performed using CST Microwave Studio software.

3 Results and Discussions

3.1 Without Directors

A summary of the reflection coefficients for the antenna rotated at different angles are shown in Fig. 3. It also includes the result of normal patch antenna without liquid metal. The normal patch antenna has resonance frequency at 5.74 GHz, which offers 2.8 dBi gain at only forward section. For the rotating antenna, due to the symmetry of the structure, its rotation at 45° and 315°, 90° and 275°, 135° and 225° resulted in the same S11. On the other hand, the S11 from the rotation at degree 0° and 180° are not identical due to the asymmetry of the patch, ground plane, and coaxial feed. The operating frequencies are between 5.71 and 5.83 GHz with the central frequency of approximately 5.8 GHz. The bandwidths of this antenna are around 330–430 MHz. The highest gain of the structure, which is 4.13 dBi, is produced when the antenna is rotated at 45° and, 135°. All results are summarized in Table 1. The radiation patterns of the xy plane (E-plane) when rotated at each angle are shown in Fig. 4. It is observed that most of the patterns maintained radiation towards the upper section, even when the antenna is rotated. When the H-plane radiation is considered, the patterns are pointed in the upper-forward direction. However, the patterns at 0° and 180° showed the lowest gains compared to the other angles due to the asymmetric position of the feed and the effect of ground plane reflection. Only at these two angles, the patterns are stronger towards the forward section than the upper section. To illustrate, Fig. 5 presents the example of H-plane radiation at the angles of 90o and 180°.

Fig. 3.
figure 3

The reflection coefficient of the antenna for various rotation angles.

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Table 1. Properties of the proposed antenna without directors
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Fig. 4.
figure 4

Simulated radiation patterns at the E-plane (Phi (deg) vs dBi) of the proposed antenna for various rotation angles; (a) 0°, (b) 45°, (c) 90°, (d) 135°, (e) 180°, (f) 225°, (g) 270°, (h) 325°.

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Fig. 5.
figure 5

Simulated radiation patterns at the H-plane (Theta (deg) vs dBi) of the proposed antenna for various rotation angles; (a) 90°, (b) 180°.

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3.2 Effect of Directors

Table 2 displays the comparison of maximum gains between antennas without directors, with corner directors (at every 90°) and with corner and side directors (at every 45°) relative to the main radiator. It is seen that all three cases resulted in similar gain values. Thus, with this dimension, the directors are slightly affected. Then, the substrate dimensions have been changed to 70 × 70mm2 with a longer (10 mm) director length. As an illustration, the example of an antenna rotating at 90° is chosen as a case study. When the substrate dimensions are larger, the resulting gains are higher, with directors changing the behavior of the corresponding patterns more significantly than a smaller substrate. This effect is shown in Table 3. However, with the addition of the directors, the major lobes of the radiation pattern in the yz plane (H-plane) become higher in the forward direction. Therefore, in such case, the antenna operating with only the cavity-filled liquid metal as a reflector is sufficient to perform beam reflection.

Table 2. Gain of the proposed antenna without directors and with different directors configurations.
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Table 3. Comparison of the proposed antenna substrates.
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4 Conclusions

A passively reconfigurable antenna using gravitational mechanism is proposed and studied in this paper. The antenna is designed on a low-cost FR4 and PDMS substrate, implementing the radiator-director concept in a Yagi-Uda antenna. Liquid metal EGain partially fills a cavity in the PDMS enclosure and is rotated to study its radiation behavior due to gravitational reconfiguration. Results indicate that the antenna operated with a consistent 330–430 MHz bandwidth centered at 5.8 GHz when rotated at eight different angles spaced at 45° when operated in a vertical configuration. Six of these eight rotation angles also produced a consistent radiation towards the upper direction despite these rotations. On the other hand, the remaining two angles of 0° and 180° mainly radiated towards the forward direction, similar as conventional patch antennas. Additional directors and a larger antenna substrate can be used to improve the directivity of these radiation patterns, but this expected improvement is not obvious due to the existence of the ground plane. Further experiments with actual implementation will be reported separately to study the practical aspects such as the flow of liquid metal on the antenna performance in the near future.