# UAV Detection Using Thrust Engine Electromagnetic Spectra
## 3 Results and Discussion
According to the obtained spectrograms from different engines, it is possible to summarize that:  
* ES signatures of electrically powered multi-rotor UAVs are received, recorded, and analyzed. * ES signatures of Dewalt impact driver are received, recorded, and analyzed. * There are enough data to conclude that ES signatures of engines from different drones and ES signature of engine from impact driver generates completely different ES signatures. * A precision tunable variable inductance magnetic antenna can be used to detect the first harmonic of the UAV's ES signature and then retune to the second or any other present harmonics. * An antenna capable of tuning harmonics can increase the effectiveness of signature detection algorithms by checking ES signatures between the different harmonics of the same UAV. Figure 10: Spectrogram of antenna with shifted frequency from 23 kHz to 46 kHz and back to 23 kHz, when 1—received UAV EM spectrum signature of the second harmonic; 2—stepless transition of magnetic antenna resonant frequency to the first harmonic of UAV EM spectrum signature; 3—received UAV EM spectrum signature of the first harmonic; 4—stepless transition of magnetic antenna resonant frequency to the second harmonic of UAV EM spectrum signature; 5—received UAV EM spectrum signature of the second harmonic. * Each forced movement of the UAV generates a unique signature in an electromagnetic spectrum that is proportional to the frequency shift of the signal. For example, the comparison of the ES signature of the Eachine Tyro drone in the spectrogram with all five harmonics in the 0-120 kHz frequency range are presented in Figure 11a. Overall, there are five harmonics at 24, 48, 72, 96, and 120 kHz frequencies, respectively. In the case of the DJI drone, there are three harmonics (see Figure 11b). The width of the different harmonics in the spectrogram of the Dewalt impact driver are compared in Figure 11c. A more detailed comparison of spectrograms of the main frequencies of the harmonics of the UAVs is presented in Tables 1-3. \begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Harmonics** & **1** & **2** & **3** & **4** & **5** \\ \hline The lower frequencies of the harmonic signature, Hz & 21,806 & 45,548 & 69,677 & 93,419 & 117,290 \\ \hline The upper frequencies of the harmonic signature, Hz & 26,064 & 49,806 & 73,806 & 97,815 & 121,548 \\ \hline The width of the harmonic signature, Hz & 4258 & 4258 & 4129 & 4396 & 4258 \\ \hline The center frequencies of the harmonic signature, Hz & 24,000 & 47,870 & 71,612 & 95,612 & 119,612 \\ \hline The amplitude of center frequency, dBm & \(-47.6\) & \(-51.2\) & \(-58.9\) & \(-64.3\) & \(-70.7\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main frequencies of the harmonics of the Eachine Tyro drone’s spectrogram. Figure 11: Spectrograms of engines with all visible harmonics and their widths in the 0–120 kHz frequency range: (**a**)—Eachine Tyro drone; (**b**)—DJI drone; (**c**)—Dewalt impact driver. The more detailed comparison of spectrograms shows that the mean width of the harmonic signature is 4259.8 Hz. The third harmonic has the narrowest width, which is equal to 4129 Hz and the fourth harmonic has the widest width, which is equal to 4396 Hz. The highest center frequency of the harmonic signature could be received in the case of the fifth harmonic--119,612 Hz and the lowest 24,000 Hz at the first harmonic (see Table 1). The three harmonics could be received in the case of the DJI drone. The mean width of the harmonic signature is 7547.6 Hz. The width of the harmonic signatures varies from 7394 Hz to 7740 Hz. The center frequencies of the harmonic signature vary from 46,280 Hz to 138,931 Hz in all three harmonics (see Table 2). The narrowest width of the harmonic signature is received in the case of the Dewalt driver, the lowest width of the harmonic signature is 1386 Hz in the case of the third harmonic. Additionally, we detected only four harmonics (see Table 3). The center frequencies of the harmonic signature vary from 20,090 Hz to 80,221 Hz.