## 2 Platform Design  
The configuration of the drone-borne antenna-measuring system is shown in Figure 1.  
When measuring HF antennas with a drone-borne acquisition system, weight and size constraints apply. Most of the commercial off-the-shelf drones can carry loads of up to 0.5 kg, and the load size should not affect the drone stability. As a result, only electrically small probes can be used, such as short straight or folded dipoles, or small loop antennas. Such antennas provide good pattern diagram stability over a wide frequency band; conversely, the radiation resistance is quite low, and the input reactance is high. Table 2 shows a comparison between three types of electrically small probes, in terms of input impedance, radiation pattern and mechanical stability. All three probes have the same first resonant frequency, i.e., 150 MHz. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Characteristics**} & \multicolumn{3}{c}{**Type of HF Antenna-Measuring System**} \\ \cline{2-4}  & **Drone-Borne Receiver** & **Drone-Borne Transmitter** & **Over Method: Drone-Borne Receiver, Near-Field** \\  & **with Active Antenna**[24] & **with Resonant Antenna**[25] & **Measurements with an** \\  & & & **Electrically Short Antenna** \\ \hline \multirow{2}{*}{Data post-processing} & No & No & \\  & (far-field measurements) & (far-field measurements) & Yes (near-field) \\ \hline Approx. power consumption & 3–5 W & 3–5 W & \textless{}1 W \\ \hline Approx. weight & 6 kg & \textless{}0.5 kg & \textless{}0.5 kg \\ \hline Approx. cost (including drone) & 15,000 \(\upxi\) & \textless{}2500 \(\upxi\) & \textless{}2500 \(\upxi\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._  
Figure 1: Drone-borne antenna-measuring system. Although the short, straight dipole has a higher radiation resistance, the folded dipole and the loop input reactance is inductive and much lower, and therefore easier to mitigate over a wide frequency band by simply connecting in parallel the balun input inductance. The loop exhibits a stable pattern diagram over the entire frequency range; however, since the main lobe lies in the loop plane, the probe should be hanged vertically below the drone in a less stable mechanical position. Reinforcing a vertical loop in order to make it less deformable and more stable results in using additional dielectric supports and/or a thicker antenna conductor, thus impacting on the overall payload and flight stability. These considerations lead to the conclusion that using an electrically short, folded dipole is a good tradeoff to achieve our goal. The dual-mode radiation of the folded dipole is analyzed in Section 3. More detailed, quantitative data on the probe used in our design including input impedance and gain are given in Section 4; we also show that the gain variation over the frequency range of interest does not impact the probe performance. For HF band measurements, non-Foster impedance-matching networks can be easily implemented [28; 29], to compensate for the antenna input reactance, and the peak detector can be designed so as to provide an appropriate input resistance. Nevertheless, using a non-Foster impedance-matching network on a drone-borne system increases the power consumption and load mass; additionally, the exposure to HF radiation might impact the active circuit operation. The most critical challenges when using a non-Foster circuit would be first the payload, and second the power consumption. The non-Foster circuitry might be neither heavy nor with a high power consumption, but those figures add to the weight and consumption of the data acquisition and communication systems. The power consumption may indirectly result in a heavier load when the battery capabilities are exceeded, and a higher-capacity battery is needed. Moving to the next payload class of drones increases the overall cost of the system by more than 100%. A simpler approach would be to terminate the probe on a constant, 50 ohm resistor without using any impedance-matching network.