Passively Reconfigurable Antenna Using Gravitational Method

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 (ε_r = 2.75) and located on a 1.6 mm-thick FR4 substrate (ε_r = 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.

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.

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.

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°.

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.