# Exploring Carbon-Fiber UAV Structures as Communication Antennas for Adaptive Relay Applications
## 4 Discussion
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Antenna Type** & **Material** & **Frequency (MHz)** & **Bandwidth (MHz)** & **Gain (dBi)** & **Size (cm)** \\ \hline Carbon-fiber Monopole (This Work) & Carbon Fiber Composite & 381 \& 1190 & 80 \& 150 & 0.15 (diameter) \\ \hline Directive Probe for Weather Radar [14] & Aluminum/Durable & 10,000 & 500 & 8 & 15 (length) \(\times\) 5 (diameter) \\ \hline Dual-band Blade [26] & Aluminum + FR-4 & 1030–1090 \& 3400–3800 & \(\approx\)60 \& 400 & 1-2 (at 1.09 GHz \& 3.5 & height 6.5 cm (\(\lambda\)/4 at 1.1 GHz) \(\times\) diameter & 1.0 cm \\ \hline Quadcopter Frame Patch [27] & Copper & 5800 & 200 & 3 & 8 \(\times\) 8 (patch) \\ \hline
4-Element Linear Array [26] & Copper & 2400 & 100 & 6 & array length 10 \(\times\) element spacing 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of UAV-Mounted Antenna Designs. Viewed alongside these alternatives, our carbon-fiber monopole is unique in achieving dual-band operation (0.381 GHz and 1.19 GHz) while adding negligible weight or aerodynamic drag. Its low peak gain (0.15 dBi) produces a broad beam that tolerates UAV motion and minor misalignments--contrasting with the narrow beams of higher-gain designs that demand precise orientation. The directive X-band probe achieves 8 dBi at 10 GHz but requires a 15 cm \(\times\) 5 cm form factor and careful pointing. The dual-band blade and 4-element array boost gain (up to 6 dBi) but incur larger footprints and heavier metallic structures. Meanwhile, the quadcopter frame patch strikes a balance at 5.8 GHz (3 dBi, 8 \(\times\) 8 cm) but cannot operate at our sub-GHz bands. For operation at 2.4 GHz, where \(\lambda/4\)\(\simeq\)3.125 cm, a compact four-element array could restore the necessary aperture area and yield improved gain and beam control. Such an array must be expressly designed and phased at 2.4 GHz; merely scaling a sub-GHz monopole design will not attain optimal performance without re-optimizing element dimensions and the matching network. Figure 25 consolidates free-space and two-ray propagation model forecasts alongside the measured S\({}_{11}\), VSWR, gain, and link-budget data. This comprehensive comparison both validates the antenna's baseline efficacy and pinpoints the targeted enhancements, return-loss realignment, impedance-matching optimization, and radiation-efficiency improvement, which will drive future UAV communication reliability. As can be seen in the graphs above, the following observations can be made: VSWR Comparison (Top-Left Plot): The blue circles with error bars represent the measured VSWR values, while the dashed line with squares represents the theoretical values at 381.45 MHz and 1.19 GHz. The error bars show a slight deviation, with the measured values being slightly higher than the theoretical ones, which suggests minor impedance mismatches or environmental factors affecting the antenna performance. The difference is small, meaning the VSWR remains within an acceptable range for practical applications. Reflection Coefficient (S11) Comparison (Top-Right Plot): This plot compares the theoretical and measured reflection coefficient (S11 in dB) across the two frequency points. The measured values (blue circles) are slightly higher than the theoretical values (dashed line with squares), meaning the antenna reflects slightly more power than expected. This  
Figure 25: Measured values compared to theoretical values. could be due to imperfections in the carbon fiber material, connection losses, or calibration errors in the measurement setup. Gain Comparison (Bottom-Left Plot): This bar chart directly compares the theoretical and measured antenna gain (dBi) at 1.2 GHz. The measured gain is slightly lower (0.15 dBi) than the theoretical gain (0.19 dBi), indicating minor efficiency losses in real-world conditions. The drop in gain may result from imperfections in the carbon fiber's conductivity or small misalignments in the measurement setup. Received Power vs.