## 4 Antennas and Antenna Related Technologies for Future Internet
### Antennas for Distributed Solutions Based on Network Structure
#### 4.4.2 Antennas for Side-Link Communications  
Several innovative antenna designs have been introduced to enhance UAV and vehicular communication systems. The quad-polarization reconfigurable conformal array antenna [304] employs a patch radiator with edge slots and shorting pins, arranged in a \(3\times 3\) subarray configuration. This design achieves a gain greater than 14 dBi and enables wide beam-scanning coverage (\(50^{\circ}\times 180^{\circ}\)), making it suitable for UAV platforms requiring flexible polarization control. The balloon conformal heterogeneous array antenna [305] integrates dumbbell-shaped and size-tapered dipoles (see Figure 19a) to achieve 360\({}^{\circ}\) azimuthal coverage and dynamic beam switching. This feature makes it highly adaptable for aerial communication scenarios. Another significant innovation is the planar dual-mode dipole antenna (Figure 19b) [306], which combines dipole and monopole radiation characteristics using H-slot-loaded microstrip arms. Operating across the 0.94-1.57 GHz range, it offers stable vertically polarized radiation patterns, ensuring reliable long-range UAV communications. For improved UAV coverage, the null-filled axisymmetric secant square-shaped beam antenna [307] employs a patch radiator with a grooved circular structure and four monopoles (Figure 19c), efficiently mitigating null formations while ensuring stable connectivity. Meanwhile, the airborne flexible rectifying metasurface system [308] utilizes metasurface structures with rectifying diodes to maximize power absorption efficiency and maintain stable wide-angle reception, enhancing UAV applications that require power harvesting capabilities. Table 7 summarizes, in more detail, various antenna characteristics for side-link communications. Figure 19: UAV antenna designs: (a) size tapered dipoles, (b) U-shaped dipoles, and (c) grooved patch. \begin{table}
\begin{tabular}{l l l l l l l} \hline \hline
**Ref.** & **Element Type** & **No. \({}^{\star}\)** & **Size (mm\({}^{3}\))** & **Frequency Band (GHz)** & **No. of Beams** & **Gain** \\ \hline
[304] & Slotted Circular Patch & 9 & \(930\times 340\times 5.8\) & 1.5 & 3 & 14 \\
[305] & Conformal Dumbbell-Shaped Dipoles & 18 & \(400\times 1462\times 55\) & 2.4 & 9 & 12 \\
[306] & H-Slot Laped Dipole & 1 & \(90\times 10\times 1\) & 0.94–1.57 & 1 & 4.76 \\
[307] & Grooved Circular Patch with Monopoles & 1 & \(225\times 225\times 2.8\) & 5.4–5.8 & 1 & 9 \\
[308] & Slotted Rectangular Patch & 16 & \(36.2\times 36.2\times 0.3\) & 5.8 & N/A & N/A \\
[309] & Monopolar Stacked Circular Patch & 1 & \(170\times 170\times 4175\) & 2.24–253, 542-5.98 & N/A & 6/7.5 \\
[310] & Hexagonal Fern-Fractal Patch & 1 & \(29.8\times 29.8\times 17.95\) & 3.49–3.9 & N/A & 5.16 \\
[311] & Circular Patch with L-Shaped Stubs & 1 & \(135\times 135\times 15.27\) & 2.6–2.98 & N/A & 5.1 \\
[312] & Hybrid Quasi-Circular and Square Monopole & 1 & \(24\times 16\times 0.8\) & 3.1–10.9 & N/A & 3.5 \\
[313] & Soft Patch & 4 & \(20\times 10\times 20\) & 3.25–6.7 & N/A & 4.97 \\
[314] & Annular Arc Monopole & 4 & \(32\times 15\times 32\) & 2.8–9.5 & N/A & 3.2–5.41 \\
[315] & Dielectric Resonator & 4 & \(100\times 100\times 50\) & 2.5–3 & N/A & 2.5–5 \\
[316] & Dielectric-Loaded Horn Antenna & 4 & \(152\times 152\times 20\) & 0.7–6 & 4 & 8–8.7 \\
[317] & F Monopole Antenna & 4 & \(50\times 50\times 33.3\) & 1.95–625 & 4 & 3.3–6.16 \\
[318] & Rectangular Patch with Lens & 1 & \(734\times 734\times 87.03\) & 5.85–5.92 & 1 & 14.3 \\
[319] & Square Monopole with Defected Ground & 32 & 142 \(\times\) 142 \(\times\) 1142.5 & 3.03–15.33 & N/A & 12.1 \\ \hline \hline \end{tabular} \({}^{\star}\) Number of elements.  
\end{table} 
Table 7: Summary of antenna characteristics for side link communications. In vehicular communication, the low-profile dual-band stacked microstrip monopolar patch antenna [309] leverages stacked microstrip patches and conductive vias to enhance bandwidth while preserving stable omnidirectional patterns.