# An Experimental Tethered UAV-Based Communication System with Continuous Power Supply
## 7 Saving the iptables rules' configuration:
#### 3.2.5 Power System Analysis and Design
This visualization enables the analysis of the system's energy consumption, as shown in Figure 18. This figure shows that the current remains stable at a value of 12.54 A, although it exhibits fluctuations due to variations in the power demands of the UAV, particularly during accelerations or decelerations, or to maintain flight stability. On the other hand, the voltage remains almost constant at around 14.73 V, indicating that the power system, comprising the battery and the voltage regulator, can handle current fluctuations efficiently, ensuring a stable power supply even when the UAV's load varies. * **Power supply selection and characteristics**: Based on the tests performed to determine the UAV power supply requirements, a voltage of 16 V and a current of 20 A were established as operating parameters. To meet these conditions, a switch-mode power supply was selected, whose specifications are presented in Table 10 and illustrated in Figure 19. Because this supply provides a higher voltage than necessary, we chose to use a step-down voltage regulator, as shown in Figure 20, to lower the voltage to 16 V. Additionally, this regulator can continuously handle up to 20 A, thus satisfying the system requirements. Figure 17: Power consumption of the UAV at 2.4 m.  
Figure 18: Consumption graph of the UAV powered by a battery, which has the Raspberry Pi and its battery as its payload. * **Calculations for cable size determination**: To select the appropriate cable, the specific energy parameters of the design are considered and are detailed below. * Voltage required by the UAV: 16 V.
* Current required: 20 A. * Distance between the power supply and the UAV: 10 m.
* Conductor material: Copper. When power is transmitted through a cable, there are losses due to the resistance of the conductor material. These losses generate a voltage drop proportional to the length of the cable, the current flowing through it, and the resistance of the conductor. For this reason, the voltage reaching the UAV will be lower than the initial voltage at the source if not adequately compensated. The National Electrical Code (NEC) [40] and IEC 60204-1 [41] state that a maximum voltage drop of 10 % is commonly accepted in power supply systems to ensure safe and efficient operation, even with long-distance cables or high currents. \begin{table}
\begin{tabular}{c c} \hline
**Feature** & **Details** \\ \hline
**Input voltage** & 110 V or 220 V \\ \hline
**Voltage range** & 0–24 V \\ \hline
**Voltage accuracy** & 0.1 V \\ \hline
**Current** & 20 A \\ \hline \end{tabular}
\end{table} 
Table 10: Technical specifications of the power supply. Figure 19: Switching power supply of 24 V and 20 A.  
Figure 20: Voltage reducer module 20 A 6V-40 VDC. Therefore, for this prototype, a maximum voltage drop of 10 % is considered. This condition ensures that the UAV receives a minimum voltage of 16 V at the cable end. Consequently, the source output voltage must be higher than the voltage required by the UAV. Equation (3) determines the calculation of the source output voltage. Thus, the source output voltage must be 17.7 V to compensate for the losses in the cable. Therefore, for this prototype, a maximum voltage drop of 10 % is considered. This ensures that the UAV receives a minimum voltage of 16 V at the cable end. Consequently, the source output voltage must be higher than the voltage required by the UAV. The Equation determines the calculation of the source output voltage must be 17.7 V to compensate for the losses in the cable. \[\lambda\times 0.9=16\,\mathrm{V} \tag{3}\]  
\[\lambda=17.7\,\mathrm{V} \tag{4}\]  
The total resistance of the cable, considering a current of 20 A and the established voltage drop of 1.7 V (17.7 V-16 V), is calculated using Ohm's Law, as shown in Equation (5). \[R=\frac{V}{I} \tag{5}\]  
\[R=\frac{1.7\,\mathrm{V}}{20\,\mathrm{A}} \tag{6}\]  
\[R=0.09\,\mathrm{\Omega} \tag{7}\]  
The next step is to determine the wire gauge required to ensure this maximum resistance using the formula in Equation (8).